Influenza vaccine effectiveness in preventing hospitalization among Beijing residents in China, 2013–15

BackgroundEstimates of influenza vaccination effectiveness (VE) are valuable for populations where the vaccine has been promoted in order to support vaccination policy and to permit evaluation of vaccination strategies. Such studies would be important for China due to limited data available during seasons when the vaccine strains matched or mismatched the circulating viruses.MethodsWe conducted a test-negative study in hospitals in Beijing. Patients admitted to five hospitals in the city were enrolled during the winter influenza seasons of 2013–14 and 2014–15. Influenza virus infections were determined by PCR, and influenza vaccination records were extracted from a centralized electronic immunization registry. Influenza VE was estimated by logistic regression adjusting for age group, sex and chronic conditions, and matched by calendar week.ResultsA total of 2368 inpatients were recruited during the study period with a vaccination coverage in the control group of 12.8%. The overall estimate of influenza VE was 46.9% (95% CI: −20.4%, 76.6%) for the 2013–14 season and 5.0% (95% CI: −53.0%, 41.0%) for the 2014–15 season. Estimates of VE were relatively higher in children aged 6–17 years than older persons across two influenza seasons while estimates of VE for both adults and elderly were relatively low.ConclusionsOur findings were consistent with expected influenza vaccination effectiveness in seasons when the vaccine matched or mismatched circulating viruses. Strategies to increase influenza vaccine coverage could provide a public health benefit.     

Estimation of oral poliovirus vaccine effectiveness in Afghanistan, 2010–2020

BackgroundAfghanistan is one of two countries with endemic wild poliovirus type 1 (WPV1). The oral poliovirus vaccine (OPV) is the predominant vaccine used for polio eradication. Although OPV has been administered in routine childhood immunization and during frequent supplementary immunization activities, WPV1 continues to circulate in Afghanistan and case incidence has been increasing since 2017. We estimated the effectiveness of OPV in Afghanistan during 2010–2020.MethodsWe conducted a matched case-control analysis using acute flaccid paralysis (AFP) surveillance data from 29,370 children < 15 years with AFP onset between January 1, 2010 and December 31, 2020. We matched children with confirmed WPV1 (cases) with children with non-polio AFP (controls) by age at onset of paralysis (+/- 3 months), date of onset of paralysis (+/- 3 months), and province of residence, and compared their reported OPV vaccination history to estimate the effectiveness of OPV in preventing paralysis by WPV1 using conditional logistic regression. To account for changes in OPV formulations provided over the analysis period, we stratified the analysis based on dates of the global switch from trivalent OPV (tOPV) to bivalent OPV (bOPV) in April 2016.ResultsBetween January 1, 2010 and December 31, 2020, there were 329 WPV1 cases in Afghanistan. The per-dose estimated effectiveness of OPV against WPV1 was 19% (95% CI: 15%–22%) and of ≥ 7 doses was 94% (95% CI: 90%-97%). Before the global switch from tOPV to bOPV, the per-dose estimated effectiveness of OPV was 14% (95% CI: 11%-18%) and of ≥ 7 doses was 92% (95% CI: 85%-96%). After the switch, the per-dose estimated effectiveness of OPV against WPV1 was 32% (24%-39%) and of ≥ 7 doses was 96% (95% CI: 90%-99%).DiscussionOPV is highly effective in preventing paralysis by WPV1; these results indicate that continued WPV1 transmission in Afghanistan is due to failure to vaccinate, not failure of the vaccine. Although difficult to implement in parts of country, improving the administration of OPV in routine immunization and supplementary immunization activities will be critical for achieving polio eradication in Afghanistan.     

The effectiveness of influenza vaccination in preventing hospitalizations in children in Hong Kong, 2009–2013

Background

Influenza vaccination is widely recommended every year to protect individuals against influenza virus infection and illness. There are few published estimates of influenza vaccine effectiveness against hospitalization in children or from subtropical regions.

Methods

We conducted a test-negative year-round study between October 2009 and September 2013, recruiting children 6 months to 17 years of age admitted to two hospitals in Hong Kong with a febrile acute respiratory infection. Cases were tested for influenza A and B and conditional logistic regression was used to estimate vaccine effectiveness comparing influenza vaccination history of the trivalent influenza vaccine (TIV) among patients testing positive versus negative for influenza, adjusting for age and sex and matching by calendar week of recruitment.

Results

Overall vaccine effectiveness against hospitalization with laboratory-confirmed influenza A and B was estimated to be 61.7% (95% CI: 43.0%, 74.2%). The estimated vaccine effectiveness against A(H3N2) was 36.6% (95% CI: −25.5%, 67.9%) compared to 71.5% (95% CI: 39.4%, 86.6%) for A(H1N1)pdm09 and 68.8% (95% CI: 41.6%, 83.3%) for B.

Conclusions

Vaccine effectiveness against hospitalization in children varied from year to year, but was moderate to high overall even in an area with influenza activity throughout the year.     

Influenza vaccine effectiveness against hospitalization with laboratory-confirmed influenza in Greece: A pooled analysis across six seasons, 2013–2014 to 2018–2019

BackgroundMonitoring seasonal influenza Vaccine Effectiveness (VE) is key to inform vaccination strategies and sustain uptake. Pooling data across multiple seasons increases precision and allows for subgroup analyses, providing more conclusive evidence. Our aim was to assess VE against hospitalization with laboratory-confirmed influenza in Greece over six seasons, from 2013 to 2014 to 2018–2019, using routinely collected surveillance data.MethodsSwab samples from hospitalized patients across the country were tested for influenza by RT-PCR. We used the test-negative design, with patients testing positive for influenza serving as cases and those testing negative serving as controls. VE was calculated as one minus the Odds Ratio (OR) for influenza vaccination, estimated by mixed-effects logistic regression and adjusted for age, sex, hospitalization type (being in intensive care or not), time from symptom onset to swabbing, and calendar time. Stratified estimates by age and hospitalization type were obtained, and also subgroup estimates by influenza type/subtype and season. Antigenic and genetic characterization of a subset of circulating influenza strains was performed.ResultsA total of 3,882 test-positive cases and 5,895 test-negative controls were analyzed. Across all seasons, adjusted VE was 45.5% (95% CI: 31.6–56.6) against all influenza, 62.8% against A(H1N1)pdm09 (95% CI: 40.7–76.7), 28.2% against A(H3N2) (95% CI: 12.0–41.3) and 45.5% against influenza B (95% CI: 29.1–58.1). VE was slightly lower for patients aged 60 years and over, and similar between patients hospitalized inside or outside intensive care. Circulating A(H1N1)pdm09 and B strains were antigenically similar to the vaccine strains, whereas A(H3N2) were not.ConclusionOur results confirm the public health benefits from seasonal influenza vaccination, despite the suboptimal effectiveness against A(H3N2) strains. Continued monitoring of VE is essential, and routinely collected surveillance data can be valuable in this regard.     

Influenza vaccine effectiveness against medically-attended influenza illness during the 2012–2013 season in Beijing,China

Background

Influenza vaccine coverage remains low in China, and there is limited information on the preventive value of local vaccination programs.

Methods

As part of influenza virological surveillance in Beijing, China during the 2012–2013 influenza season, we assessed the vaccine effectiveness (VE) of one or more doses of trivalent inactivated influenza vaccine (IIV3) in preventing medically-attended influenza-like-illness (ILI) associated with laboratory-confirmed influenza virus infection using a test-negative case–control design. Influenza vaccination was determined based on self-report by adult patients or the parents of child patients.

Results

Of 1998 patients with ILI, 695 (35%) tested positive for influenza viruses, including 292 (42%) A(H3N2), 398 (57%) A(H1N1)pdm09, and 5 (1%) not (sub)typed influenza viruses. The rate of influenza vaccination among all patients was 4% (71/1998). Among influenza positive patients, 2% (57/1303) were vaccinated compared to 4% (14/695) among influenza negative patients, resulting in VE for one or more doses of vaccine (adjusted for age, sex, week, and days since illness onset) against all circulating influenza viruses of 52% (95% CI = 12–74%). A significant adjusted VE for one or more doses of vaccine for all ages against A(H1N1)pdm09 of 59% (95% CI, 8–82%) was observed; however, the VE against A(H3N2) was 43% (95% CI, −30% to 75%). The point estimate of VE was 59% (95% CI, 19–79%) for those aged <60 years, but a negative VE point estimate without statistical significance was observed among those aged ≥60 years.

Conclusions

IIV3 conferred moderate protection against medically-attended influenza in Beijing, China during the 2012–2013 season, especially against the A(H1N1)pdm09 strain and among those aged <60 years old.     

Three-season effectiveness of inactivated influenza vaccine in preventing influenza illness and hospitalization in children in Japan, 2013–2016

ObjectivesWe assessed the vaccine effectiveness (VE) of inactivated influenza vaccine (IIV) in children 6 months to 15 years of age in 2015/16 season. In addition, based on the data obtained during the three seasons from 2013 to 2016, we estimated the three-season VE in preventing influenza illness and hospitalization.MethodsOur study was conducted according to a test-negative case-control design (TNCC) and as a case-control study based on influenza rapid diagnostic test results.ResultsDuring 2015/16 season, the quadrivalent IIV was first used in Japan. The adjusted VE in preventing influenza illness was 49% (95% confidence interval [CI]: 42–55%) against any type of influenza, 57% (95% CI: 50–63%) against influenza A and 34% (95% CI: 23–44%) against influenza B. The 3-season adjusted VE was 45% (95% CI: 41–49%) against influenza virus infection overall (N = 12,888), 51% (95% CI: 47–55%) against influenza A (N = 10,410), and 32% (95% CI: 24–38%) against influenza B (N = 9232). An analysis by age groups showed low or no significant VE in infants or adolescents. By contrast, VE was highest in the young group (1–5 years old) and declined with age thereafter. The 3-season adjusted VE in preventing hospitalization as determined in a case-control study was 52% (95% CI: 42–60%) for influenza A and 28% (95% CI: 4–46%) for influenza B, and by TNCC design, it was 54% (95% CI: 41–65%) for influenza A and 34% (95% CI: 6–54%) for influenza B.ConclusionWe demonstrated not only VE in preventing illness, but also VE in preventing hospitalization based on much larger numbers of children than previous studies.     

Comparison of vaccine effectiveness against influenza hospitalization of cell-based and egg-based influenza vaccines, 2017–2018

Egg-based influenza vaccines could be less effective than cell-based vaccine due to adaptive mutations acquired for growth. We conducted a test-negative case-control study at Kaiser Permanente Southern California to assess vaccine effectiveness (VE) against hospitalization for laboratory-confirmed influenza during 2017–2018. Among the 1186 cases and 6946 controls, 74% and 59%, respectively, were ages ≥ 65 years. For any influenza, the adjusted relative VE of cell-based vaccine versus egg-based vaccines was 43% (95% CI: −45% to 77%) for patients ages < 65 years and 6% (95% CI: −46% to 39%) for patients ages ≥ 65 years. For influenza A(H3N2), the adjusted relative VE was 61% (95% CI: −63% to 91%) for patients ages < 65 years and −4% (95% CI: −70% to 37%) for patients ages ≥ 65 years. Statistically significant protection against influenza hospitalization of cell-based vaccine compared to egg-based vaccines was not observed, but further studies in additional influenza seasons are warranted.     

Declining Influenza Vaccination Coverage among Nurses,Hong Kong, 2006–2012

Seasonal influenza vaccination of nurses in Hong Kong fell from 57% in 2005 to 24% in 2012, paralleling concern for adverse reactions associated with vaccination. Decreased acceptance of vaccination was most prominent among nurses who had less work experience and more frequent contact with patients.     

Influenza vaccines effectiveness 2013–14 through 2015–16, a test-negative study in children

BackgroundTrivalent inactivated and live attenuated influenza vaccines (IIV3 and LAIV3) have been reformulated with an extra B strain (IIV4 and LAIV4). They were licensed based on immunogenicity and their effectiveness (VE) still must be empirically tested.MethodsChildren 1–17 years tested for influenza during 2013–16 were included and their immunization status verified. They were considered vaccinated if received ≥1 dose of an influenza vaccine ≥10 days before evaluated for a respiratory episode. Age-groups were classified as 1–4 years or 5–17 years. VE was estimated by comparing vaccination status of influenza-positive versus influenza-negative cases.Results6779 children were enrolled in the three seasons. Overall, 27.2% received an influenza vaccine (87.1% IIV3 or IIV4 and 12.9% LAIV4), and 15.6% tested positive for influenza (77.9% A). IIV3 was predominantly used in 2013–14 and IIV4 in 2014–15 and 2015–16. IIV3 and IIV4 had comparable VE over the three seasons (60%, 57% and 53%) and performed similarly against influenza A and B and both age-groups. LAIV4 performed poorly for influenza A (15%, 37% and 48%) but better for influenza B (100%, 56% and 100%), especially among children 5–17 years of age with VE = 100% (95%CI: 55, 100).ConclusionsInfluenza vaccination showed modest but consistent effectiveness over the years. The switch from IIV3 to IIV4 did not affect VE. LAIV4 did not perform as well as IIVs, yet it improved over the years and was particularly good protecting older children against influenza B. These results emphasize the regional nature of influenza and the need for local surveillance.     

Influenza vaccine effectiveness against laboratory confirmed influenza in Greece during the 2013–2014 season: A test-negative study

BackgroundIn 2013–2014 Greece experienced a resurgence of severe influenza cases, coincidental with a shift to H1N1pdm09 predominance. We sought to estimate Vaccine Effectiveness (VE) for this season using available surveillance data from hospitals (including both inpatients and outpatients).MethodsSwab samples were sent by hospital physicians to one of three laboratories, covering the entire country, to be tested for influenza using RT-PCR. The test-negative design was employed, with patients testing positive serving as cases and those testing negative serving as controls. VE was estimated using logistic regression, adjusted for age group, sex, region and calendar time, with further adjustment for unknown vaccination status using inverse response propensity weights. Additional age group stratified estimates and subgroup estimates of VE against H1N1pdm09 and H3N2 were calculated.ResultsOut of 1310 patients with known vaccination status, 124 (9.5%) were vaccinated, and 543 patients (41.5%) tested positive for influenza. Adjusted VE was 34.5% (95% CI: 4.1–55.3%) against any influenza, and 56.7% (95% CI: 22.8–75.7%) against H1N1pdm09. VE estimates appeared to be higher for people aged 60 and older, while in those under 60 there was limited evidence of effectiveness. Isolated circulating strains were genetically close to the vaccine strain, with limited evidence of antigenic drift.ConclusionsThese results suggest a moderate protective effect of the 2013–2014 influenza vaccine, mainly against H1N1pdm09 and in people aged 60 and over. Vaccine coverage was very low in Greece, even among groups targeted for vaccination, and substantial efforts should be made to improve it. VE can and should be routinely monitored, and the results taken into account when deciding on influenza vaccine composition for next season.     

Influenza vaccine effectiveness among patients with high-risk medical conditions in the United States, 2012–2016

BackgroundAnnual influenza vaccination has been recommended for persons with high-risk conditions since the 1960s. However, few estimates of influenza vaccine effectiveness (VE) for persons with high-risk conditions are available.MethodsData from the U.S. Influenza Vaccine Effectiveness Network from 2012 to 2016 were analyzed to compare VE of standard-dose inactivated vaccines against medically-attended influenza among patients aged ≥6 months with and without high-risk medical conditions. Patients with acute respiratory illness were tested for influenza by RT-PCR. Presence of high-risk conditions and vaccination status were obtained from medical records. VE by influenza virus type/subtype and age group was calculated for patients with and without high-risk conditions using the test-negative design. Interaction terms were used to test for differences in VE by high-risk conditions.ResultsOverall, 9643 (38%) of 25,369 patients enrolled during four influenza seasons had high-risk conditions; 2213 (23%) tested positive for influenza infection. For all ages, VE against any influenza was lower among patients with high-risk conditions (41%, 95% CI: 35–47%) than those without (48%, 95% CI: 43–52%; P-for-interaction = 0.02). For children aged <18 years, VE against any influenza was 51% (95% CI: 39–61%) and 52% (95% CI: 39–61%) among those with and without high-risk conditions, respectively (P-for-interaction = 0.54). For adults aged ≥18 years, VE against any influenza was 38% (95% CI: 30–45%) and 44% (95% CI: 38–50%) among those with and without high-risk conditions, respectively (P-for-interaction = 0.21). For both children aged <18 and adults aged ≥18 years, VEs against illness related to influenza A(H3N2), A(H1N1)pdm09, and influenza B virus infection were similar among those with and without high-risk conditions.ConclusionsInfluenza vaccination provided protection against medically-attended influenza among patients with high-risk conditions, at levels approaching those observed among patients without high-risk conditions. Results from our analysis support recommendations of annual vaccination for patients with high-risk conditions.     

Influenza vaccine effectiveness against hospitalization during the 2018/2019 season among older persons aged ≥ 75 years in Japan: The LIFE-VENUS Study

BackgroundOlder persons are recommended to receive annual influenza vaccinations due to their increased susceptibility to influenza infections and related complications. Routine assessments of influenza vaccine effectiveness (IVE) in older persons may help to improve vaccine development and vaccination strategies, but there is a lack of consistent epidemiological data from Japan. This study aimed to evaluate IVE against hospitalization during the 2018/2019 season among older persons aged ≥ 75 years in Japan.MethodsThis cohort study was conducted using insurance claims data and vaccination records provided by the Longevity Improvement & Fair Evidence - Vaccine Effectiveness, Networking, and Universal Safety (LIFE-VENUS) Study. The study cohort comprised older persons aged ≥ 75 years residing in an urban municipality in Japan. Vaccinated participants were identified through vaccination records from October 2018 to January 2019, and were matched with unvaccinated participants using a 1:1 ratio. The IVE against hospitalization was calculated as (1?hazard ratio) × 100% while adjusting for covariates such as age, sex, comorbidities, previous vaccinations, and care needs levels.ResultsWe analyzed 30,881 vaccinated participants matched with 30,881 unvaccinated participants. Among these, 587 (1.9%) vaccinated participants and 644 (2.1%) unvaccinated participants were hospitalized during the 2018/2019 season. The adjusted IVE against hospitalization was estimated to be 28.9% (16.6–39.4%).ConclusionsThe influenza vaccine for the 2018/2019 season showed moderate effectiveness among older persons in Japan. The LIFE-VENUS Study represents a potential platform for the continued monitoring of IVE among the older Japanese population.     

Influenza vaccine effectiveness: Maintained protection throughout the duration of influenza seasons 2010–2011 through 2013–2014

BackgroundFactors, such as age, comorbidities, vaccine type, herd immunity, previous influenza exposure, and antigenic shift may impact the immune response to the influenza vaccine, protection against circulating strains, and antibody waning. Evaluating vaccine effectiveness (VE) is important for informing timing of vaccine administration and evaluating overall vaccine benefit.MethodsVE was assessed using febrile respiratory illness surveillance among Department of Defense non-active duty beneficiaries from influenza seasons 2010–2011 through 2013–2014. Respiratory specimens were taken from participants meeting the case definition and tested by polymerase chain reaction for influenza. VE was calculated using logistic regression and by taking 1 minus the odds ratio of being vaccinated in the laboratory confirmed positive influenza cases versus laboratory confirmed negative controls.ResultsThis study included 1486 participants. We found an overall adjusted VE that provided significant and fairly consistent protection ranging from 54% to 67% during 0–180 days postvaccination. This VE dropped to −11% (95% confidence interval: −102% to 39%) during 181–365 days.ConclusionsOur study found moderate VE up to 6 months postvaccination. Since the influenza season starts at different times each year, optimal timing is difficult to predict. Consequently, early influenza vaccination may still offer the best overall protection.     

Assessment of influenza vaccine effectiveness in a sentinel surveillance network 2010–13, United States

BackgroundInfluenza vaccines are now widely used to reduce the burden of annual epidemics of influenza virus infections. Influenza vaccine effectiveness (VE) is monitored annually to determine VE against each season's circulating influenza strains in different groups such as children, adults and the elderly. Few prospective surveillance programs are available to evaluate influenza VE against medically attended illness for patients of all ages in the United States.MethodsWe conducted surveillance of patients with acute respiratory illnesses in 101 clinics across the US during three consecutive influenza seasons. We analyzed laboratory testing results for influenza virus, self-reported vaccine history, and patient characteristics, defining cases as patients who tested positive for influenza virus and controls as patients who tested negative for influenza virus. Comparison of influenza vaccination coverage among cases versus controls, adjusted for potential confounders, was used to estimate VE as one minus the adjusted odds ratio multiplied by 100%.ResultsWe included 10,650 patients during three influenza seasons from August 2010 through December 2013, and estimated influenza VE in children 6m–5y of age (58%; 95% CI: 49%–66%), children 6–17y (45%; 95% CI: 34%–53%), adults 18–49y (36%; 95% CI: 24%, 46%), and adults ≥50y (34%, 95% CI: 13%, 51%). VE was higher against influenza A(H1N1) compared to A(H3N2) and B.ConclusionsOur estimates of moderate influenza VE confirm the important role of vaccination in protecting against medically attended influenza virus infection.     

Seasonal influenza vaccine effectiveness against influenza in 2012–2013: A hospital-based case-control study in Lithuania

Background

Due to scarce information on seasonal influenza vaccine effectiveness (SIVE) against severe clinical influenza outcomes in risk populations, we conducted a case-control study to assess its effects against laboratory-confirmed influenza in hospitalized patients during the 2012–2013 influenza season.

Methods

We conducted a test-negative case-control study among ≥18 years old patients with influenza-like illness (ILI) hospitalized in two Lithuanian hospitals. Cases were influenza A(H1N1), A(H3) or influenza B positive by RT-PCR, and controls were influenza negative. Additional demographic and clinical data to assess the role of confounding were collected. SIVE and its confidence intervals (95% CI) were estimated by using multivariate logistic regression as (1 − OR) × 100%.

Results

The sample consisted of 185 subjects. Seasonal influenza vaccine uptake was 5%. Among 111 (60%) influenza positive cases, 24.3% were A(H1N1), 10.8% were A(H3) and 24.3% were influenza B cases. Unadjusted SIVE was 79% (95% CI −6% to 96%) and after the adjustment it increased to 86% (95% CI 19% to 97%).

Conclusions

Seasonal influenza vaccination in 2012–2013 was associated with reduced occurrence of laboratory-confirmed influenza, but due to low sample size the estimate of SIVE is imprecise. Given high prevalence of influenza in hospitalized ILI cases and low influenza vaccination coverage, there is a need to increase influenza vaccination rates.     

Effectiveness of inactivated influenza vaccine in children by vaccine dose, 2013–18

ObjectivesWe assessed the vaccine effectiveness (VE) of inactivated influenza vaccine (IIV) by vaccine dose in children aged 6 months to 12 years for whom two doses are recommended in Japan to ascertain the appropriate vaccine doses.MethodsVE was assessed according to a test-negative case-control design based on rapid influenza diagnostic test (RIDT) results. Children aged 6 months to 12 years with a fever ≥38 °C who had received an RIDT in outpatient clinics of 24 hospitals were enrolled for all five seasons since 2013/14. VE by vaccine dose (none vs. once or twice, and once vs. twice) was analyzed.ResultsIn the dose analysis, 20,033 children were enrolled. Both one- and two-dose regimens significantly reduced cases in preventing any influenza, influenza A, and influenza B, but there was no significant difference in adjusted VE between one- and two-dose regimens overall (adjusted OR, 0.560 [95% CI, 0.505–0.621], 0.550 [95% CI, 0.516–0.586]), 0.549 [95% CI, 0.517–0.583], and 1.014 [95% CI, 0.907–1.135], for none vs. once, none vs. twice, none vs. once or twice, and once vs. twice for any influenza, respectively). Both one- and two-dose regimens significantly reduced cases with any influenza and influenza A every season. Also, both regimens significantly reduced cases of any influenza, influenza A, and influenza B among children aged 1–12 years, especially among those aged 1–5 years. In the 2013/14, 2015/16, and 2016/17 seasons, however, only the two-dose regimen was significantly effective in preventing influenza B. Both one- and two-dose regimens significantly reduced cases involving hospitalization due to any influenza and influenza A.ConclusionsBoth one- and two-doses regimens of IIV were effective in preventing influenza for children aged 6 months to 12 years. The two-dose regimen was more effective against influenza B in some seasons.     

Estimated COVID-19 vaccine effectiveness against seroconversion from SARS-CoV-2 Infection,March–October, 2021

BackgroundMonitoring the effectiveness of COVID-19 vaccines against SARS-CoV-2 infections remains important to inform public health responses. Estimation of vaccine effectiveness (VE) against serological evidence of SARS-CoV-2 infection might provide an alternative measure of the benefit of vaccination against infection.MethodsWe estimated mRNA COVID-19 vaccine effectiveness (VE) against development of SARS-CoV-2 anti-nucleocapsid antibodies in March–October 2021, during which the Delta variant became predominant. Participants were enrolled from four participating healthcare systems in the United States, and completed electronic surveys that included vaccination history. Dried blood spot specimens collected on a monthly basis were analyzed for anti-spike antibodies, and, if positive, anti-nucleocapsid antibodies. We used detection of new anti-nucleocapsid antibodies to indicate SARS-CoV-2 infection, and estimated VE by comparing 154 case-participants with new detection of anti-nucleocapsid antibodies to 1,540 seronegative control-participants matched by calendar period. Using conditional logistic regression, we estimated VE ≥ 14 days after the 2nd dose of an mRNA vaccine compared with no receipt of a COVID-19 vaccine dose, adjusting for age group, healthcare worker occupation, urban/suburban/rural residence, healthcare system region, and reported contact with a person testing positive for SARS-CoV-2.ResultsAmong individuals who completed a primary series, estimated VE against seroconversion from SARS-CoV-2 infection was 88.8% (95% confidence interval [CI], 79.6%–93.9%) after any mRNA vaccine, 87.8% (95% CI, 75.9%–93.8%) after BioNTech vaccine and 91.7% (95% CI, 75.7%–97.2%) after Moderna vaccine. VE was estimated to be lower ≥ 3 months after dose 2 compared with < 3 months after dose 2, and among participants who were older or had underlying health conditions, although confidence intervals overlapped between subgroups.ConclusionsVE estimates generated using infection-induced antibodies were consistent with published estimates from clinical trials and observational studies that used virologic tests to confirm infection during the same period. Our findings support recommendations for eligible adults to remain up to date with COVID-19 vaccination.