Serologic Responses in Healthy Adult with SARS-CoV-2 Reinfection,Hong Kong,August 2020

In March 2020, mild signs and symptoms of coronavirus disease developed in a healthy 33-year-old man in Hong Kong. His first infection did not produce virus neutralizing antibodies. In August, he had asymptomatic reinfection, suggesting that persons without a robust neutralizing antibody response might be at risk for reinfection.     

Durability of Antibody Response and Frequency of SARS-CoV-2 Infection 6 Months after COVID-19 Vaccination in Healthcare Workers

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies decay but persist 6 months postvaccination; lower levels of neutralizing titers persist against Delta than wild-type virus. Of 227 vaccinated healthcare workers tested, only 2 experienced outpatient symptomatic breakthrough infections, despite 59/227 exhibiting serologic evidence of SARS-CoV-2 infection, defined as presence of nucleocapsid protein antibodies.     

Limited and Short-Lasting Virus Neutralizing Titers Induced by Inactivated SARS-CoV-2 Vaccine

In vitro determination of severe acute respiratory syndrome coronavirus 2 neutralizing antibodies induced in serum samples from recipients of the CoronaVac vaccine showed a short protection period against the original virus strain and limited protection against variants of concern. These data provide support for vaccine boosters, especially variants of concern circulate.     

SARS-CoV-2 Omicron BA.1 Challenge after Ancestral or Delta Infection in Mice

We assessed cross-reactivity to BA.1, BA.2, and BA.5 of neutralizing antibodies elicited by ancestral, Delta, and Omicron BA.1 SARS-CoV-2 infection in mice. Primary infection elicited homologous antibodies with poor cross-reactivity to Omicron strains. This pattern remained after BA.1 challenge, although ancestral- and Delta-infected mice were protected from BA.1 infection.     

Persistence of Antibodies against Middle East Respiratory Syndrome Coronavirus

To determine how long antibodies against Middle East respiratory syndrome coronavirus persist, we measured long-term antibody responses among persons serologically positive or indeterminate after a 2012 outbreak in Jordan. Antibodies, including neutralizing antibodies, were detectable in 6 (86%) of 7 persons for at least 34 months after the outbreak.     

Lack of MERS Coronavirus Neutralizing Antibodies in Humans,Eastern Province,Saudi Arabia

We used a lentiviral vector bearing the viral spike protein to detect neutralizing antibodies against Middle East respiratory syndrome coronavirus (MERS-CoV) in persons from the Eastern Province of Saudi Arabia. None of the 268 samples tested displayed neutralizing activity, which suggests that MERS-CoV infections in humans are infrequent in this province.     

Genomic Evidence of SARS-CoV-2 Reinfection Involving E484K Spike Mutation,Brazil

Uncertainty remains about how long the protective immune responses against severe acute respiratory syndrome coronavirus 2 persists, and suspected reinfection in recovered patients has been reported. We describe a case of reinfection from distinct virus lineages in Brazil harboring the E484K mutation, a variant associated with escape from neutralizing antibodies.     

Cross-Variant Neutralizing Serum Activity after SARS-CoV-2 Breakthrough Infections

To determine neutralizing activity against the severe acute respiratory syndrome coronavirus 2 ancestral strain and 4 variants of concern, we tested serum from 30 persons with breakthrough infection after 2-dose vaccination. Cross-variant neutralizing activity was comparable to that after 3-dose vaccination. Shorter intervals between vaccination and breakthrough infection correlated with lower neutralizing titers.     

Prevalence of SARS-CoV-2 Antibodies after First 6 Months of COVID-19 Pandemic,Portugal

In September 2020, we tested 13,398 persons in Portugal for antibodies against severe acute respiratory syndrome coronavirus 2 by using a quota sample stratified by age and population density. We found a seroprevalence of 2.2%, 3–4 times larger than the official number of cases at the end of the first wave of the pandemic.     

Improving ability of RSV microneutralization assay to detect G-specific and cross-reactive neutralizing antibodies through immortalized cell line selection

Respiratory syncytial virus (RSV) is a significant cause of bronchiolitis and pneumonia. Protection against RSV is associated with neutralizing antibodies against the fusion (F) and attachment (G) glycoproteins. Several RSV vaccine candidates are in development, but their immunogenicity is hard to compare due to the little-understood differences between multiple RSV neutralizing antibody assays used. Existing assays utilize primarily Vero or HEp-2 cells, but their ability to detect G-neutralizing antibodies or antibodies against specific RSV strains is unclear. In this work, we developed an RSV microneutralization assay (MNA) using unmodified RSV and immortalized cell line derived from human airway epithelial cells (A549). Performance of A549-, HEp-2- and Vero-based MNA was compared under the same assay conditions (fixed amount of virus and cells) with regards to detection of neutralizing antibodies against RSV A or B viruses, G-reactive neutralizing antibodies, and effect of complement. Our results indicate that A549 cells yield the highest MNA titers, particularly in the RSV A/A2 MNA, are least susceptible to complement-enhancing effect of neutralizing titer readout and are superior to Vero or HEp-2 MNA at recognizing G-reactive neutralizing antibodies when no complement is used. Vero cells, however, can be more consistent at recognizing neutralizing antibodies against multiple RSV strains. The choice of substrate cells thus affects the outcome of MNA, as some immortalized cells better support detection of broader range of neutralizing antibodies, while others facilitate detection of G-targeting neutralizing antibodies, a long-thought prerogative of primary airway epithelial cells.     

Antibodies against MERS Coronavirus in Dromedary Camels,United Arab Emirates, 2003 and 2013

Middle East respiratory syndrome coronavirus (MERS-CoV) has caused an ongoing outbreak of severe acute respiratory tract infection in humans in the Arabian Peninsula since 2012. Dromedary camels have been implicated as possible viral reservoirs. We used serologic assays to analyze 651 dromedary camel serum samples from the United Arab Emirates; 151 of 651 samples were obtained in 2003, well before onset of the current epidemic, and 500 serum samples were obtained in 2013. Recombinant spike protein–specific immunofluorescence and virus neutralization tests enabled clear discrimination between MERS-CoV and bovine CoV infections. Most (632/651, 97.1%) camels had antibodies against MERS-CoV. This result included all 151 serum samples obtained in 2003. Most (389/651, 59.8%) serum samples had MERS-CoV–neutralizing antibody titers >1,280. Dromedary camels from the United Arab Emirates were infected at high rates with MERS-CoV or a closely related, probably conspecific, virus long before the first human MERS cases.     

Persistence of SARS-CoV-2–Specific IgG in Children 6 Months After Infection,Australia

The duration of the humoral immune response in children infected with severe acute respiratory syndrome coronavirus 2 is unknown. We detected specific IgG 6 months after infection in children who were asymptomatic or had mild symptoms of coronavirus disease. These findings will inform vaccination strategies and other prevention measures.     

Enhancing neutralizing antibody production by an interferon-inducing porcine reproductive and respiratory syndrome virus strain

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) continues to cause substantial economic losses to the global swine industry. PRRSV appears to inhibit synthesis of type I interferons (IFNs), such as IFN-α and -β, which are critical for the innate immunity and play an important role in the modulation of adaptive immunity. An atypical PRRSV strain, A2MC2, is able to induce type I IFNs in vitro. In this study, A2MC2 induction of neutralizing antibodies in vivo was compared with the Ingelvac PRRS modified live virus (MLV) vaccine strain and VR-2385 (a moderate virulent strain). Three-week-old pigs were exposed to these PRRSV strains via intranasal or intramuscular routes to also account for a possible effect of inoculation routes. The interferon-inducing A2MC2 resulted in earlier onset and significantly higher levels of PRRSV neutralizing antibodies than the MLV. In addition, the A2MC2-induced neutralizing antibodies were capable of neutralizing VR-2385, a heterologous strain. The pigs exposed via intranasal route had higher titers of neutralizing antibodies than those injected via intramuscular route. Macroscopic and microscopic lung lesions 14 days post-exposure indicated that A2MC2 had similar virulence in vivo as VR-2385. Pulmonary alveolar macrophages (PAMs) collected during the necropsy 14 days post-exposure in the A2MC2 group had higher level expression of IFN-γ than the MLV group. These results indicate that A2MC2 can be further explored for development of an improved vaccine against PRRS.     

Genetic Evidence and Host Immune Response in Persons Reinfected with SARS-CoV-2, Brazil

The dynamics underlying severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reinfection remain poorly understood. We identified a small cluster of patients in Brazil who experienced 2 episodes of coronavirus disease (COVID-19) in March and late May 2020. In the first episode, patients manifested an enhanced innate response compared with healthy persons, but neutralizing humoral immunity was not fully achieved. The second episode was associated with different SARS-CoV-2 strains, higher viral loads, and clinical symptoms. Our finding that persons with mild COVID-19 may have controlled SARS-CoV-2 replication without developing detectable humoral immunity suggests that reinfection is more frequent than supposed, but this hypothesis is not well documented.     

Evaluation of Commercially Available High-Throughput SARS-CoV-2 Serologic Assays for Serosurveillance and Related Applications

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serosurveys can estimate cumulative incidence for monitoring epidemics, requiring assessment of serologic assays to inform testing algorithm development and interpretation of results. We conducted a multilaboratory evaluation of 21 commercial high-throughput SARS-CoV-2 serologic assays using blinded panels of 1,000 highly characterized specimens. Assays demonstrated a range of sensitivities (96%–63%), specificities (99%–96%), and precision (intraclass correlation coefficient 0.55–0.99). Durability of antibody detection was dependent on antigen and immunoglobulin targets; antispike and total Ig assays demonstrated more stable longitudinal reactivity than antinucleocapsid and IgG assays. Assays with high sensitivity, specificity, and durable antibody detection are ideal for serosurveillance, but assays demonstrating waning reactivity are appropriate for other applications, including correlation with neutralizing activity and detection of anamnestic boosting by reinfections. Assay performance must be evaluated in context of intended use, particularly in the context of widespread vaccination and circulation of SARS-CoV-2 variants.     

Establishment of stable mouse/human-human hybrid cell lines producing large amounts of anti-tetanus human monoclonal antibodies with high neutralizing activity

To establish stable hybrid cell lines producing human anti-tetanus antibody with high toxinneutralizing activity, peripheral lymphocytes from humans hyperimmunized with tetanus toxoid were, after in vitro antigen stimulation, fused with a mouse/human heteromyeloma or human lymphoblastoid cell line and cloned. Unlike the IgM secretors (six clones), the IgG secretors we obtained (six clones) produced anti-tetanus human monoclonal antibodies with high neutralizing activity (the highest one, cell line G2, 4.3 IU/100 g IgG).Appropriate combinations of three or four kinds of monoclonal antibodies of the IgG type resulted in markedly increased neutralizing activity comparable with that of anti-tetanus human polyclonal immunoglobulin preparations currently used clinically on the basis of toxin-specific IgG content. Five of these cell lines produced 1020 pg of antibody per ml for more than 3 months. The cell line G2 produced 6 mg of antibody per day in serum-free medium in a 500-ml bioreactor in perfusion culture and 13–104 mg in a nude mouse. These cell lines satisfied, for the first time, the minimal requirements for applying human monoclonal antibodies to clinical use.     

Possible Human-to-Dog Transmission of SARS-CoV-2, Italy, 2020

We detected severe acute respiratory syndrome coronavirus 2 in an otherwise healthy poodle living with 4 family members who had coronavirus disease. We observed antibodies in serum samples taken from the dog, indicating seroconversion. Full-length genome sequencing showed that the canine and human viruses were identical, suggesting human-to-animal transmission.     

Cross-Sectional Serosurvey of Companion Animals Housed with SARS-CoV-2–Infected Owners,Italy

We conducted a serologic survey among dogs and cats in Italy to detect antibodies against severe acute respiratory syndrome virus 2 (SARS-CoV-2). We found that SARS-CoV-2 seroprevalence was higher among cats (16.2%) than dogs (2.3%). In addition, seroprevalence was higher among animals living in close contact with SARS-CoV-2–positive owners.     

Antibody Responses 8 Months after Asymptomatic or Mild SARS-CoV-2 Infection

Waning humoral immunity in coronavirus disease patients has raised concern over usefulness of serologic testing. We investigated antibody responses of 58 persons 8 months after asymptomatic or mildly symptomatic infection with severe acute respiratory syndrome coronavirus 2. For 3 of 4 immunoassays used, seropositivity rates were high (69.0%–91.4%).