The use of homologous virus in the haemagglutination-inhibition assay after vaccination with Newcastle disease virus strain La Sota or Clone30 leads to an over estimation of protective serum antibody titres.

We evaluated the influence of the use of the Newcastle disease virus (NDV)-strains Ulster and La Sota in the haemagglutination inhibition (HI) assay for the measurement of antibody titres after NDV vaccination. The use of the homologous La Sota antigen in the HI assay after Clone30 and La Sota vaccination of SPF-chickens, resulted in significantly higher titres than the use of the heterologous Ulster virus. The mean difference was 1.4 on a log 2 scale (2.6-fold). A significant difference was also found in virus neutralization (VN) assays. The virus strain in the HI or VN test did not influence the resulting titres after Ulster vaccination. When HI antibody titres after vaccination were related to VN titres measured with virulent Herts NDV or to survival after virulent challenge, it was found that the use of La Sota antigen in the HI assay after vaccination with Clone30 or La Sota resulted in an overestimation of protective serum antibody titres. Also in commercially derived White Leghorns vaccinated with Clone30, significantly higher HI titres were obtained when La Sota antigen was used in the HI test. Our data have direct implications for potency testing of inactivated vaccines as the European Pharmacopeia does not prescribe the antigen to be used in the HI test.     

Levels of immunoglobulins and antibodies to haemaglutinin and neuraminidase of influenza virus in nasal secretions after natural infection.

Nasal washings (NW) from 16 influenza patients in the course of an epidemic in November and December, 1974 were examined for the presence of influenza virus, immunoglobulins (Ig) and titres of haemagglutination inhibiting (HI) and neuraminidase inhibiting (NI) antibodies. Influenza virus identical with A/Port Chalmers/1/73 (H3N2), increased levels of IgA and occasionally IgG, and specific antibodies were detected in the NW. The dynamics of HI and NI antibody formation did not differ substantially, but there were individual differences in titres and persistence of antibodies. Convalescent sera always contained increased levels of HI and NI antibodies. In some cases, the titres of antibody to viral ribonucleoprotein did not increase.     

Response of Specific Skin Hypersensitivity and Haemagglutination Inhibiting Antibody after Smallpox Vaccination in Human Newborns and Adults

We tried to assess the response of the specific cell-mediated and humoral immunity after primary smallpox vaccination in normal full term newborns and compared these results with those found in revaccinated adults. Revaccination and intradermal inoculation of heat-inactivated vaccinia virus were the two methods employed to study the specific delayed hypersensitivity against this virus.
We found that, although these newborn babies were able to produce haemagglutination inhibiting (HI) antibody even more efficiently than the adults, the skin hypersensitivity developed in them after primary vaccination was of much lower extent than in the revaccinated adults. The HI antibody response in the vaccinated newborns was initially 2-ME sensitive (100%), while in the revaccinated adults the early HI antibodies were mixed varieties, i.e. both 2-ME sensitive (38.6%) and 2-ME resistant (61.4%); then there was a gradual shift towards 2-ME resistant HI antibody in the newborns. In adults the humoral antibody consisted of both 2-ME resistant and sensitive antibodies already at 1 week. The cord sera contained only 2-ME resistant HI antibody which had a half life of 14 days in the unvaccinated newborn babies. In the vaccinated group of newborns there was an initial quicker elimination of IgG than in the unvaccinated babies. In addition, seroconversion was negligible in those babies whose cord sera had an initially very high HI antibody titre. Immunological implications of these findings have been discussed.     

Efficacy of inactivated infectious bursal disease (IBD) vaccines: Comparison of serology with protection of progeny chickens against IBD virus strains of varying virulence

The efficacy of inactivated infectious bursal disease vaccines was determined by measuring both the antibody response of vaccinated chickens and clinical protection of progeny chicks from vaccinated dams. Similar virus neutralizing (VN) antibody titres were obtained in 4-week-old chickens and mature hens after vaccination with one vaccine dose. VN titres below 10 log 2 increased considerably between the fourth and seventh week after vaccination in 4-week-old chickens as well as in mature chickens. All 2-week-old progeny chicks with serum VN antibody titres of at least 9 log 2 were clinically protected against the classical virulent 52/70 infectious bursal disease virus (IBDV) strain, as well as against the very virulent IBDV (vvIBDV) strain D6948. However, vaccination often did not prevent subclinical infection in these 2-week-old progeny chicks, which often resulted in severe lymphocyte depletion in the bursa of Fabricius. Even a serum VN titre of 11 log 2 was not always sufficient to prevent severe bursal damage. Although 52/70 IBDV and vvIBDV were equally pathogenic in 2-weekold specific pathogen free chickens, significant higher maternal antibody titres were required to prevent the adverse effects of vvIBDV in comparison with 52/70 IBDV. The relation between the serological response of chickens after application of inactivated IBD vaccines and the protection of progeny chicks of vaccinated dams depended on both the virulence of the IBDV challenge strain and the IBDV strain in the vaccine.     

Differences in antibody responses of mice to intranasal or intraperitoneal immunization with influenza A virus and vaccination with subunit influenza vaccine

Two antigenically related but different influenza A virus strains of H3N2 subtype, A/Dunedin/ 4/73 (H3N2) (Dunedin) and A/Mississippi/1/85 (H3N2) (Mississippi), were used for intranasal (i.n.) and intraperitoneal (i.p.) immunization of mice and respective antibody responses were compared. In ELISA, using purified influenza A virus as antigen, the highest titer of antiviral antibodies was observed after a repeated i.n. infection, in which the Dunedin strain was followed by the Mississippi strain and vice versa. Similarly, in virus neutralization (VN) test, the highest titer of VN antibodies was found after a repeated i.n. infection. The subunit vaccine INFLUVAC, when administered intramuscularly (i.m.), induced only a poor antibody response as assayed by ELISA. Moreover, the INFLUVAC vaccination elicited a 100-fold lower titer of VN antibodies than the i.n. infection and an approx a 10-fold lower titer than the i.p. immunization. A repeated INFLUVAC vaccination did not lead to a significant increase of VN antibody titer. Also the antibody response to HA2gp--a conserved part of influenza hemagglutinin (HA) that might contribute to the induction of specific antiviral antibodies--was followed. Similarly to the VN antibody response, the highest HA2 antibody titer was induced after a repeated i.n. infection, whereas the lowest HA2 antibody titer was observed after a single or repeated INFLUVAC vaccination. Overall, the HA2 antibody titers remarkably well corresponded to the VN potential of the examined sera.     

Attenuated influenza A vaccine (Alice) in an adult population: vaccine-related illness, serum and nasal antibody production, and intrafamily transmission.

Ninety-five healthy adults, ages 18 to 56 years, received two intranasal doses, 2 weeks apart, of a live, attenuated, influenza type A (H3N2) vaccine (an inhibitor-resistant recombinant strain of A/England/42/72 named "Alice"). Ninety-two persons were given placebos similarly. Ninety-three percent of 68 subjects with initial serum hemagglutination-inhibition (HI) titers of greater than or equal to 1:40 to influenza A (H3N2) had a fourfold or greater antibody increase in postvaccination sera. Forty-four percent of 27 subjects with an initial HI titer of greater than or equal to 1:80 had similar increases. Overall, 77% of vaccinees had fourfold or greater antibody titer increases. Vaccinees had geometric mean serum HI titers (GMT) of 1:26, 1:123, and 1:166 at 0, 14, and 30 days, respectively. The GMTs for placebos were 1:21, 1:22, and 1:21. Thirty-five vaccinees were examined for both serum and nasal antibody; 89% had significant increases in one or both. Nasal antibody response was directly related to the level of initial serum HI titer in that 83% of 12 persons with prevaccination HI titers of 1:80 greater than or equal to 1:80 showed significant nasal antibody rises, whereas only 61% of the remaining 23 subjects with prevaccination HI titers of less than or equal to 1:40 did so. The number and severity of clinical signs and symptoms reported by vaccinees and placebos did not differ significantly. The greatest differences noted between groups were for nasal congestion on days 0 to 6 (8.3%) and rhinitis on days 14 to 20 (5.9%). Four vaccinees shed Alice after primary vaccination, but viral titers were low (10 to 100 tissue culture-infective doses/ml). One member in each of 15 cohabiting male-female couples received Alice while the other received a placebo; one of the placebo members had significant increases in serum and nasal antibody, indicating a possible transmission.     

Immunogenicity of a monovalent, aluminum-adjuvanted influenza whole virus vaccine for pandemic use

In the case of an influenza pandemic a significant gap between influenza vaccine manufacturing capacities and vaccine demands must be expected on a global scale. This study explores the possibility to increase the number of vaccine doses that can be provided with the existing resources by using a lower amount of antigen per dose in an aluminum-adjuvanted whole virus vaccine formulation, instead of the standard dosage of 15 microg hemagglutinin (HA). The study was performed as an open, non-controlled, randomized, multicentric study in 200 volunteers (18-30 years of age). Three monovalent aluminum-adjuvanted whole virus formulations with different antigen concentrations (1.9, 3.75 and 7.5 microg HA per dose) were compared to a split virus vaccine (15 microg HA per dose) without aluminum adjuvantation. The sera were tested for hemagglutination inhibition (HI) antibodies, neuraminidase inhibition (NI) antibodies and virus neutralizing (VN) antibodies. Nasal swab samples were tested for influenza-specific IgA antibodies. All volunteers were immunologically na?ve to the vaccine strain influenza A/Singapore/1/57 (H2N2). The vaccine was well tolerated. HI titers reached protective levels (geometric mean titer (GMT) >1:40) after two vaccine doses. In the group immunized with the lowest antigen dose the seroprotection rate was 82%. Although the immune response tends to be lower for vaccine formulations with reduced antigen content, the immunogenicity criteria as defined by the European Agency for the Evaluation of Medicinal Products (EMEA) were met with all antigen formulations after two vaccine doses. Significant increases in HI, NI and VN titers were observed, however, no significant local immune response was detected. The use of a low-dose whole virus influenza vaccine, adjuvanted with aluminum appears to be a viable approach to increase vaccine supplies in a pandemic situation.     

Antibody responses to serogroup B meningococcal outer membrane antigens after vaccination and infection.

Antibody responses of adult volunteers given a vaccine containing meningococcal capsular polysaccharides (serogroups A, C, Y, and W-135) noncovalently complexed with serotype 2b:P1.2 and 15:P1.16 outer membrane proteins have been studied. Sera were analyzed by enzyme-linked immunosorbent assay methods for immunoglobulin G (IgG), IgM, and IgA antibodies and for bactericidal activities against the homologous strains. The vaccination was performed as a double-blind experiment with 47 volunteers, of whom 23 received the protein-polysaccharide vaccine and 24 received the control preparation containing the polysaccharides only. Ten additional persons volunteered for the protein-polysaccharide vaccine. Before vaccination, carriers of meningococci had significantly higher levels of specific IgG and IgA and also higher bactericidal activities than noncarriers. At 2 weeks postvaccination we found significant IgG and bactericidal antibody responses against both the 2b:P1.2 and 15:P1.16 strains in about 70% of the protein-polysaccharide vaccinees. The immune response induced by disease was compared with that induced by vaccination by analyzing paired sera from 13 survivors of serogroup B serotype 15 meningococcal disease. We found that the mean specific IgG level in acute-phase sera was lower than average in prevaccination sera from the vaccinees but similar to that of healthy noncarriers before vaccination. The convalescent-phase sera showed IgG responses similar to those of the vaccinees, but the IgM response to disease was significantly higher than after vaccination. The immune response to disease caused by serogroup B serotype 15 meningococci was found by enzyme-linked immunosorbent assay analysis to be about the same with outer-membrane antigens from a serotype 2b strain as it was with antigens from a serotype 15 strain.     

Rituximab impairs immunoglobulin (Ig)M and IgG (subclass) responses after influenza vaccination in rheumatoid arthritis patients

Rituximab (RTX) treatment in rheumatoid arthritis (RA) patients severely hampers humoral response after influenza vaccination as determined by haemagglutination inhibition assay (HI). It is not known whether HI reflects both immunoglobulin (Ig)M and IgG (subclass) influenza response, and whether IgM antibodies contribute to the low rate of influenza infection seen in RA patients. Twenty RA patients on methotrexate (MTX), 23 on RTX and 28 healthy controls (HC) received trivalent influenza subunit vaccination. Before and 28 days after vaccination, H1N1‐ and H3N2‐specific antibodies were measured by HI and by IgM and IgG (subclass) enzyme‐linked immunosorbent assay (ELISA). B cell activating factor (BAFF) levels were determined in serum samples before vaccination. Vaccination induced a significant increase of IgM and IgG (IgG1 and IgG3) antibodies against both strains in the HC and MTX groups (all P < 0·01), but not in the RTX group. HI correlated significantly in all cases with IgG (IgG1) but not with IgM. In RTX late patients (RTX treatment 6–10 months before vaccination), IgG (IgG1 and IgG3) response to vaccination was restored, but not IgM response. BAFF levels were significantly increased in RA‐RTX patients and correlated with total IgG levels. Haemagglutination inhibition assay, used as gold standard, detects primarily IgG (IgG1) responses. IgM‐ and IgG influenza‐specific antibodies increase after vaccination in HC and RA patients except in patients on RTX treatment. BAFF levels are increased in both early and late RTX‐treated patients, but do not correlate with an influenza‐specific antibody response.     

Serological responses in volunteers to inactivated trivalent subunit influenza vaccine: antibody reactivity with epidemic influenza A and B strains and evidence of a rapid immune response

A study of the immunogenicity of the inactivated trivalent subunit influenza vaccine for the 1989/90 season was performed in what proved to be an influenza epidemic year. One hundred student volunteers at The London Hospital Medical College participated in the study and the findings indicated that there was an excellent serological match between the epidemic strain of influenza A (H3N2) and the vaccine strain. Before vaccination, the geometric mean titre (GMT) to A/England/308/89, a representative H3N2 epidemic strain in the United Kingdom from the 1989/90 season, was 46. Post-vaccination the antibody levels rose and 99% of vaccinees had HI titres of greater than or equal to 40, the GMT being 131. The serological responses were also investigated against other circulating influenza A (H3N2 and H1N1) and B strains. Preliminary results of an evaluation of the rapidity of the immune response showed that in three of six subjects rises in HI antibody appeared within two days of vaccination.     

Bile immunoglobulin of the duck (Anas platyrhynchos). II. Antibody response in influenza A virus infections

The capacity of the IgM-like bile immunoglobulin (IgX) of the duck (Anas platyrhynchos) to express antibody activity to H3N2 influenza A viruses, and the dependence of this activity on the co-existence of serum IgM antibodies were investigated. Ducklings infected orally and intranasally at 15-29 days of age with viruses isolated from different host species were examined for haemagglutination-inhibiting (HI) antibodies in biles and sera 16-29 days after infection (p.i.). All biles had antibodies associated with IgX; all sera had antibodies associated only with the 7.8S IgG. Following oral infection of birds 42-days-old with influenza A/duck/HK/7/75 virus, serum HI antibodies were an initial IgM response occurring from 5-12 days p.i., followed by the appearance of 7.8S IgG antibodies. Virus-neutralizing (VN) antibodies in serum were also biphasic; isotype classification was not attempted. Bile IgX developed HI and VN activity. HI antibodies reached peak titres 12 days p.i. and fell to low levels by 24 days p.i. VN antibodies also reached peak titres 12 days p.i., but thereafter persisted at quite high levels throughout the experiment. Development of high titres of antibody in bile coincided with the termination of virus excretion in faeces. These experiments confirm that bile IgX of the duck can function as antibody in response to influenza A viruses, and that its activity appears to be independent of serum IgM. Its possible relevance in determining survival of virus in the intestine is discussed.     

Comparison of the enzyme-linked immunosorbent assay (elisa), haemagglutination inhibition test and agar gel precipitation test for detection of antibodies to avian infectious bronchitis virus

The immune response after vaccination with infectious bronchitis virus (IBV) under field conditions was measured by the ELISA, haemagglu-tination-inhibition (HI) and agar-gel precipitin (AGP), tests. Vaccinations were performed in three flocks and one experimental group via the drinking water with the vaccine strains H 120 and H 52. In each flock 40 random serum samples were taken every 2 weeks and tested individually. In the experimental group blood samples were collected every week from each of the 10 chickens. The primary vaccination with H 120 resulted in a rapid increase of antibody titre as detected by ELISA followed by a slow decrease over the next few weeks. By the HI and AGP tests no antibody responses could be seen after this primary vaccination. Revaccination with the H 52 strain provoked a further increase in ELISA titres. In the experimental group, and in flock W, a similar increase occurred by the HI test and precipitating antibodies appeared. The formation of HI antibodies in flock T (nipple waterers) was somewhat retarded and precipitating antibodies were just detectable. In flock F revaccination did not result in the immediate production of HI and AGP antibodies. However, 6 weeks after revaccination a significant rise in ELISA, HI and AGP antibodies was observed, probably as the result of a field infection. It was demonstrated that, based on the higher sensitivity, the ELISA test is more suitable than HI and AGP to monitor antibody responses to vaccination against infectious bronchitis. Strain specificity in the HI test is discussed as a reason for its failure to detect antibodies after primary vaccination with the highly attenuated vaccine strain H 120.     

Herd immunity to Newcastle disease virus in poultry by vaccination.

Newcastle disease is an economically important disease of poultry for which vaccination is applied as a preventive measure in many countries. Nevertheless, outbreaks have been reported in vaccinated populations. This suggests that either the vaccination coverage level is too low or that vaccination does not provide perfect immunity, allowing the virus to spread in partially vaccinated populations. Here we study the requirements of an epidemiologically effective vaccination program against Newcastle disease in poultry, based on data from experimental transmission studies. The transmission studies indicate that vaccinated birds with low or undetectable antibody titres may be protected against disease and mortality but that infection and transmission may still occur. In fact, our quantitative analyses show that Newcastle disease virus is highly transmissible in poultry with low antibody titres. As a consequence, herd immunity can only be achieved if a high proportion of birds (>85%) have a high antibody titre (log(2) haemagglutination inhibition titre > or =3) after vaccination. We discuss the implications for the control of Newcastle disease in poultry by vaccination.     

Enumeration of haemagglutinin-specific CD8+ T cells after influenza vaccination using MHC class I peptide tetramers

With emergence of MHC class I tetramers loaded with CD8+ T-cell viral epitopes, it is possible to study virus-specific CD8 cells in humans during infection and after vaccination. MHC class I tetramers was used to detect the frequency of haemagglutinin (HA)-specific T cells in 26 healthy influenza-vaccinated humans. Peripheral blood was collected before, and 7, 14 and 28 days after vaccination. Four-colour flow cytometry was used for monitoring of vaccine induced T-cell response. In 15 donors, two- to fivefold increase in frequency of HA-specific T cells was observed 7 days after vaccination. In addition, in 12 of these donors, this increase was accompanied with fourfold increase of H1N1 antibody titre. The increase in frequency of HA-specific CD8+/IFN-gamma+ cells was low and peaked 28 days after vaccination in three of the six donors tested. Frequencies of HA-specific CD8+ T cells and antibody titre returned to prevaccination values 1 year after vaccination. Subunit influenza vaccines have the ability to induce HA-specific CD8+ cells. As the immune response to this vaccine decreased significantly after 1 year, our results confirm the importance of annual immunization for adequate protection.     

Increased sensitivity and reduced specificity of hemagglutination inhibition tests with ether-treated influenza B/Singapore/222/79

Hemagglutination inhibition (HI) tests against whole virus (WV) influenza B/Singapore/222/79 antigen detected prevaccination serum antibody in only 15 (20%) of 50 predominantly elderly volunteers and fourfold or greater titer rises in only three (6%) after they received 1981-1982 trivalent influenza vaccine containing antigens of this virus. HI titers against ether-treated (ET) B/Singapore/222/79 were about eightfold higher than those against WV antigen and were comparable to microneutralization titers against this virus. The ET HI detected prevaccination antibody in 84%, a postvaccination titer rise in 32%, and a final titer of 80 or higher in 66%. Among 51 additional persons with known or presumed influenza B virus infections early in 1982, ET B/Singapore/222/79 was also more sensitive than WV for serodiagnosis (69 versus 49%), but eight persons with both WV and ET B/Singapore/222/79 HI responses also had an HI titer rise to WV A/Brazil/11/78 (H1N1) antigen. Conversely, among 14 college students with febrile, culture-proven influenza A (H1N1) infections early in 1982, 6 (43%) developed HI titer rises to ET B/Singapore/222/79 with no other serological evidence of influenza B virus infection. Moreover, young adult volunteers with mild experimental influenza A (H1N1) infections also exhibited a 17% (3 of 18) incidence of ET B/Singapore/222/79 HI titer rises, versus none in matched, uninfected volunteers. These data indicate that ET B/Singapore/222/79 virus has increased sensitivity but reduced specificity compared to WV as an HI antigen and that caution is needed in interpretation of a single HI test for serodiagnosis, whether with WV or ET antigen.     

Rubella reimmunization: comparative analysis of the immunoglobulin G response to rubella virus vaccine in previously seronegative and seropositive individuals.

Rubella virus (RV)-specific immunoglobulin G (IgG) antibodies were studied in military recruits undergoing unselected immunization with live attenuated measles, mumps, and rubella virus (MMR) vaccine. Three different whole-RV enzyme immunoassays (EIAs) and an epitope-specific EIA with a synthetic peptide (BCH-178c) representing a heutralization domain on the RV E1 envelope protein were used. Before vaccination, 84.2, 87.7, and 84.5% of the subjects tested (n = 399) were found to be seropositive (> 10 IU/ml or assay equivalent) by the three whole-RV EIAs, respectively, while only 82.5% were seropositive by the BCH-178c EIA. Although prevaccination seropositivity rates were similar for the whole-RV EIAs (sensitivity, 94 to 100%), many sera considered seropositive by the whole-RV EIAs had E1 peptide EIA antibody levels of < 10 IU/ml (sensitivity, 77.4 to 80.7%). One month after vaccination, 97.8, 97.2, and 93.5% of the subjects who were followed (n = 356) were seropositive by the three whole-RV EIAs, respectively, while 89% had BCH-178c peptide-specific IgG titers of > 10 IU/ml. After vaccination, depending on the assay used, up to 20.6% of initially seropositive individuals exhibited a greater than fourfold increase in RV-specific IgG, while up to 47.3% showed a greater than twofold increase. Increased antibody titers after vaccination (seroboosting) were most frequently associated with low levels of BCH-178c peptide-specific IgG before vaccination. RV protein-specific IgG was also studied by immunoblot assays in a subset (n = 56) of individuals receiving the MMR vaccine. Of these, 89.4 and 91.1% exhibited RV protein (E1, E2, and C protein)-specific IgG before and after vaccination, respectively. Seroboosting (two- to fourfold increase in EIA titers of individuals seropositive by the whole-RV EIA before vaccination) was usually accompanied by a shift in the IgG immunoblot pattern from a single (E1) to multiple (E1-E1, E1-C, or E1-E2-C) specificities, suggesting exposure of new epitopes as a result of viral replication.     

Comparative immune responses of patients with chronic pulmonary diseases during the 2-year period after pneumococcal vaccination

Antibody responses to a 23-valent pneumococcal vaccine for Streptococcus pneumoniae serotypes 6B, 14, 19F, and 23F in 84 patients with chronic pulmonary diseases over a 2-year period after vaccination were examined by using a third-generation enzyme-linked immunosorbent assay. Of these patients, 28 (31%) were low responders who had developed increases of at least twofold in the levels of serotype-specific immunoglobulin G (IgG) in sera for none of the four serotypes at 1 month after vaccination. Although no specific clinical features of low responders were evident, their prevaccination levels of IgG for all serotypes were higher than those of responders. In responders, the levels of IgG specific for serotypes 14 and 23F in sera were greatly increased 1 month after vaccination and those specific for serotypes 6B and 19F were moderately increased. In contrast, no significant increases in the levels of IgG specific for serotypes 6B, 19F, and 23F in the low responders during the same period were found, but the levels of IgG specific for serotype 14 did increase. Although a rapid decline in the levels of IgG for all serotypes in responders between 1 month and 6 months after vaccination was found, the levels of IgG specific for serotypes 14 and 23F in sera remained higher than the prevaccination levels for at least 2 years after vaccination. These data suggest the need for the revaccination of responders but not low responders among patients with chronic pulmonary diseases. Revaccination as early as 3 years postvaccination is recommended for responders to increase the reduced levels of IgG in sera, especially those specific for the weak vaccine antigens.     

Correlation between ELISA,hemagglutination inhibition,and neutralization tests after vaccination against tick-borne encephalitis

The significance of IgG antibody levels determined by a binding assay (ELISA) was investigated as a surrogate marker for the presence of neutralizing and hemagglutination inhibiting antibodies in sera from individuals vaccinated against tick-borne encephalitis (TBE). To assess the extent of interference by flavivirus cross-reactive antibodies, sera from persons with a proven or suspected history of other flavivirus infections and/or vaccinations were also examined. An excellent and highly significant correlation was found between ELISA IgG units and the antibody titers obtained by the hemagglutination inhibition (HI) as well as by the neutralization test (NT), provided that there was no other exposure to flavivirus antigens except TBE vaccination. Yellow fever vaccination and/or dengue virus infections induced significant levels of antibodies reactive in the TBE ELISA and HI test, which did not exhibit, however, neutralizing activity against TBE virus. The phenomenon and problem of “original antigenic sin” was demonstrated in a TBE vaccinee with a history of previous flavivirus infections. TBE vaccination first induced a booster reaction resulting in a rise in the level of cross-reactive antibodies only, whereas TBE virus-neutralizing antibodies became detectable only after the third vaccination. It is concluded that the level of IgG antibodies determined by ELISA is a good marker for predicting the presence of neutralizing antibodies after TBE vaccination, but only in the absence of flavivirus cross-reactive antibodies. Otherwise, a neutralization assay is necessary for assessing immunity. © 1996 Wiley-Liss, Inc.     

Serological profiles after consecutive experimental infections of pigs with European H1N1, H3N2, and H1N2 swine influenza viruses

Swine influenza viruses (SIVs) of H1N1, H3N2, and H1N2 subtypes, with antigenically different hemagglutinins, are currently cocirculating in pigs in Europe. This study aimed to determine whether the hemagglutination inhibition (HI) test, which is the primary serological test for SIV, is sufficiently specific to discriminate between infections with the three subtypes. In experiment 1, pigs were consecutively inoculated with European H1N1, H3N2, and H1N2 SIVs by the intranasal route, or with the respective subtypes only. In a second experiment, a commercial, inactivated H1N1- and H3N2- based SIV vaccine was administered once to pigs previously infected with one to three SIV subtypes or to influenza-naive pigs. Sequential serum samples were examined in HI and virus-neutralizing (VN) tests to the three strains used for pig inoculations. Of the 160 sera collected after infection with one or two SIV subtypes, only 8 showed cross-reactive antibodies to the remaining subtype(s) in the HI test, and 11 in the VN test. Consecutive inoculations with H1N1 and H1N2 or vice versa were followed by a significant rise in preexisting antibody titers to the first subtype after the second inoculation. When dually infection-immune pigs were inoculated with the third, remaining SIV subtype, nasal virus excretion was undetectable or reduced and the serological response was absent to moderate. A single vaccination of infection-immune pigs resulted in a dramatic rise in HI and VN antibody titers to any of the previously encountered subtypes, whereas SIV-naive pigs barely seroconverted. Most important, pigs previously infected with H1N1 but not with H1N2 developed crossreactive antibodies to H1N2 after the vaccination. In conclusion, the HI test remains adequate for the differential diagnosis of infections with H1N1, H3N2, and H1N2 in European swine populations if it is properly used and if the SIV vaccination status is taken into account.     

Measles immunity and response to revaccination of a young adult population in Israel

In order to evaluate the true immune status and the effect of revaccination on a young adult population, we collected serum samples from 289 military recruits who were vaccinated during an outbreak in 1991. Most vaccinees, age 18–25 years, had apparently been immunized once before as infants. Sera collected just prior to the vaccination and 14 and 28 days afterwards were tested for measles antibodies by hemagglutination inhibition (HI) and enzyme-linked immunosorbent assay (ELISA)-IgM. Before vaccination, 46 (15.9%) of the subjects had no HI antibodies, (<1:4) and 48 (16.6%) had borderline (1:4) HI titer. Following vaccination, only ten (3.5%) remained negative and 19 (6.6%) had borderline titer. The increase in HI antibody titer was inversely proportional to the prevaccination titer, and 159 subjects (55.0%) showed no increase at all. The geometric mean titer (GMT) rose from 9.14 to 21.47. Among the prevaccination-negative subjects (HI <1:4) 28 (60.9%) reached a postvaccination titer of ≥ 1:8, and eight (17.4%) reached a titer of 1:4. Twelve (26.1%) of the negative subjects seroconverted and developed IgM, 16 (35%) seroconverted without IgM, and 18 (39%) remained negative and did not develop IgM. A group of eight vaccinees with prevaccination titer of ≥ 1:4 developed IgM. Some were probably infected by the circulating wild-type virus prior to the vaccination. Thus, a total number of 20 of the 289 subjects studied (6.9%) had true negative preimmune status as judged by the IgM test. However, the vaccination campaign prevented further measles cases, apparently by increasing the population's immunity, particularly in individuals with very low titers or without measles antibodies. © 1996 Wiley-Liss, Inc.