Factors influencing estimated effectiveness of COVID-19 vaccines in non-randomised studies

Population-Based Influenza Vaccine Effectiveness Against Laboratory-Confirmed Influenza Infection in Southern China, 2023-2024 Season

Background

In China, the 2022-2023 influenza season began earlier and was characterized by higher levels of influenza activity and co-circulation of various respiratory pathogens compared with seasons before the coronavirus disease 2019 (COVID-19) pandemic. Timely and precise estimates of influenza vaccine effectiveness (IVE) against infections can be used to guide public health measures.

Methods

A test-negative study was conducted to estimate IVE against laboratory-confirmed influenza using data from the CHinese Electronic health Records Research in Yinzhou (CHERRY) study that prospectively integrated laboratory, vaccination, and health administrative data in Yinzhou, southern China. We included patients who presented influenza-like illness and received nucleic acid tests and/or antigen tests between October 2023 and March 2024. Estimates of IVE were adjusted for age, gender, month of specimen submitted, chronic comorbidities, and hospitalization status.

Results

A total of 205 028 participants, including 96 298 influenza cases (7.6% vaccinated) and 108 730 influenza-negative controls (13.4% vaccinated), were eligible for this analysis. The estimates of IVE were 49.4% (95% CI, 47.8%-50.9%), 41.9% (95% CI, 39.8%-44.0%), and 59.9% (95% CI, 57.9%-61.9%) against overall influenza, influenza A, and influenza B, respectively. A lower IVE was observed for individuals aged 7-17 years (38.6%), vs 45.8% for 6 months-6 years, 46.7% for 18-64 years, and 46.1% for ≥65 years. Vaccination reduced the risk of infection by 44.4% among patients with chronic comorbidities. IVEs varied by epidemic weeks with the changes in influenza activity levels and the switch of dominant influenza strains.

Conclusions

Influenza vaccination in the 2023-2024 season was protective against infection for the entire population.

Reports of Batch-Dependent Suspected Adverse Events of the BNT162b2 mRNA COVID-19 Vaccine: Comparison of Results from Denmark and Sweden

Background and Objective: An unexpected batch-dependent safety signal for the BNT162b2 mRNA COVID-19 vaccine was recently identified in a nationwide study from Denmark, but the generalizability of this finding is unknown. Therefore, we compared batch-dependent rates of suspected adverse events (SAEs) reported to national authorities in Denmark and Sweden. Materials and Methods: SAE and vaccine batch data were received from national authorities in Denmark and Sweden, and analyses of heterogeneity in the relationship between numbers of vaccine doses and SAEs per batch were performed, along with comparison of SAE rates and severities for batches that were shared between the two countries. Results: Significant batch-dependent heterogeneity was found in the number of SAEs per 1000 doses for both countries, with batches associated with high SAE rates detected in the early phase of the vaccination campaign and positive correlations observed between the two countries for the severity of SAEs from vaccine batches that they shared. Mild SAEs predominated in the batches used in the early part of the vaccination roll-out, where markedly higher SAE rates per 1000 doses in Denmark for the batches that were shared between the two countries suggested that a large proportion of these SAEs were under-reported in Sweden. Conclusions: The batch-dependent safety signal observed in Denmark and now confirmed in Sweden suggests that early commercial batches of BNT162b2 may have differed from those used later on, and these preliminary and hypothesis-generating results warrant further study.

Evidence base for yearly respiratory virus vaccines: Current status and proposed improved strategies

Annual vaccination is widely recommended for influenza and SARS-CoV-2. In this essay, we analyse and question the prevailing policymaking approach to these respiratory virus vaccines, especially in the United States. Every year, licensed influenza vaccines are reformulated to include specific strains expected to dominate in the season ahead. Updated vaccines are rapidly manufactured and approved without further regulatory requirement of clinical data. Novel vaccines (i.e. new products) typically undergo clinical trials, though generally powered for clinically unimportant outcomes (e.g. lab-confirmed infections, regardless of symptomatology or antibody levels). Eventually, the current and future efficacy of influenza and COVID-19 vaccines against hospitalization or death carries considerable uncertainty. The emergence of highly transmissible SARS-CoV-2 variants and waning vaccine-induced immunity led to plummeting vaccine effectiveness, at least against symptomatic infection, and booster doses have since been widely recommended. No further randomized trials were performed for clinically important outcomes for licensed updated boosters. In both cases, annual vaccine effectiveness estimates are generated by observational research, but observational studies are particularly susceptible to confounding and bias. Well-conducted experimental studies, particularly randomized trials, are necessary to address persistent uncertainties about influenza and COVID-19 vaccines. We propose a new research framework which would render results relevant to the current or future respiratory viral seasons. We demonstrate that experimental studies are feasible by adopting a more pragmatic approach and provide strategies on how to do so. When it comes to implementing policies that seriously impact people's lives, require substantial public resources and/or rely on widespread public acceptance, high evidence standards are desirable.

Effectiveness of COVID-19 vaccines administered in the 2023 autumnal campaigns in Europe: Results from the VEBIS primary care test-negative design study, September 2023-January 2024

In autumn 2023, European vaccination campaigns predominantly administered XBB.1.5 vaccine. In a European multicentre study, we estimated 2023 COVID-19 vaccine effectiveness (VE) against laboratory-confirmed symptomatic infection at primary care level between September 2023 and January 2024. Using a test-negative case-control design, we estimated VE in the target group for COVID-19 vaccination overall and by time since vaccination. We included 1057 cases and 4397 controls. Vaccine effectiveness was 40 % (95 % CI: 26-53 %) overall, 48 % (95 % CI: 31-61 %) among those vaccinated < 6 weeks of onset and 29 % (95 % CI: 3-49 %) at 6-14 weeks. Our results suggest that COVID-19 vaccines administered to target groups during the autumn 2023 campaigns showed clinically significant effectiveness against laboratory-confirmed, medically attended symptomatic SARS-CoV-2 infection in the 3 months following vaccination. A longer study period will allow for further variant-specific COVID-19 VE estimates, better understanding decline in VE and informing booster administration policies.

COVID-19 Vaccine Effectiveness in Autumn and Winter 2022 to 2023 Among Older Europeans

Importance

In the context of emerging SARS-CoV-2 variants or lineages and new vaccines, it is key to accurately monitor COVID-19 vaccine effectiveness (CVE) to inform vaccination campaigns.

Objective

To estimate the effectiveness of COVID-19 vaccines administered in autumn and winter 2022 to 2023 against symptomatic SARS-CoV-2 infection (with all circulating viruses and XBB lineage in particular) among people aged 60 years or older in Europe, and to compare different CVE approaches across the exposed and reference groups used.

Design, Setting, and Participants

This case-control study obtained data from VEBIS (Vaccine Effectiveness, Burden and Impact Studies), a multicenter study that collects COVID-19 and influenza data from 11 European sites: Croatia; France; Germany; Hungary; Ireland; Portugal; the Netherlands; Romania; Spain, national; Spain, Navarre region; and Sweden. Participants were primary care patients aged 60 years or older with acute respiratory infection symptoms who were recruited at the 11 sites after the start of the COVID-19 vaccination campaign from September 2022 to August 2023. Cases and controls were defined as patients with positive and negative, respectively, reverse transcription-polymerase chain reaction (RT-PCR) test results.

Exposures

The exposure was COVID-19 vaccination. The exposure group consisted of patients who received a COVID-19 vaccine during the autumn and winter 2022 to 2023 vaccination campaign and 14 days or more before symptom onset. Reference group included patients who were not vaccinated during or in the 6 months before the 2022 to 2023 campaign (seasonal CVE), those who were never vaccinated (absolute CVE), and those who were vaccinated with at least the primary series 6 months or more before the campaign (relative CVE). For relative CVE of second boosters, patients receiving their second booster during the campaign were compared with those receiving 1 booster 6 months or more before the campaign.

Main Outcomes and Measures

The outcome was RT-PCR-confirmed, medically attended, symptomatic SARS-CoV-2 infection. Four CVE estimates were generated: seasonal, absolute, relative, and relative of second boosters. CVE was estimated using logistic regression, adjusting for study site, symptom onset date, age, chronic condition, and sex.

Results

A total of 9308 primary care patients were included, with 1687 cases (1035 females; median [IQR] age, 71 [65-79] years) and 7621 controls (4619 females [61%]; median [IQR] age, 71 [65-78] years). Within 14 to 89 days after vaccination, seasonal CVE was 29% (95% CI, 14%-42%), absolute CVE was 39% (95% CI, 6%-60%), relative CVE was 31% (95% CI, 15% to 44%), and relative CVE of second boosters was 34% (95% CI, 18%-47%) against all SARS-CoV-2 variants. In the same interval, seasonal CVE was 44% (95% CI, -10% to 75%), absolute CVE was 52% (95% CI, -23% to 82%), relative CVE was 47% (95% CI, -8% to 77%), and relative CVE of second boosters was 46% (95% CI, -13% to 77%) during a period of high XBB circulation. Estimates decreased with time since vaccination, with no protection from 180 days after vaccination.

Conclusions and Relevance

In this case-control study among older Europeans, all CVE approaches suggested that COVID-19 vaccines administered in autumn and winter 2022 to 2023 offered at least 3 months of protection against symptomatic, medically attended, laboratory-confirmed SARS-CoV-2 infection. The effectiveness of new COVID-19 vaccines against emerging SARS-CoV-2 variants should be continually monitored using CVE seasonal approaches.

A Critical Analysis of All-Cause Deaths during COVID-19 Vaccination in an Italian Province

Immortal time bias (ITB) is common in cohort studies and distorts the association estimates between the treated and untreated. We used data from an Italian study on COVID-19 vaccine effectiveness, with a large cohort, long follow-up, and adjustment for confounding factors, affected by ITB, with the aim to verify the real impact of the vaccination campaign by comparing the risk of all-cause death between the vaccinated population and the unvaccinated population. We aligned all subjects on a single index date and considered the "all-cause deaths" outcome to compare the survival distributions of the unvaccinated group versus various vaccination statuses. The all-cause-death hazard ratios in univariate analysis for vaccinated people with 1, 2, and 3/4 doses versus unvaccinated people were 0.88, 1.23, and 1.21, respectively. The multivariate values were 2.40, 1.98, and 0.99. Possible explanations of this trend of the hazard ratios as vaccinations increase could be a harvesting effect; a calendar-time bias, accounting for seasonality and pandemic waves; a case-counting window bias; a healthy-vaccinee bias; or some combination of these factors. With 2 and even with 3/4 doses, the calculated Restricted Mean Survival Time and Restricted Mean Time Lost have shown a small but significant downside for the vaccinated populations.

COVID-19 vaccine effectiveness against symptomatic infection with SARS-CoV-2 BA.1/BA.2 lineages among adults and adolescents in a multicentre primary care study, Europe, December 2021 to June 2022

BackgroundScarce European data in early 2021 suggested lower vaccine effectiveness (VE) against SARS-CoV-2 Omicron lineages than previous variants.AimWe aimed to estimate primary series (PS) and first booster VE against symptomatic BA.1/BA.2 infection and investigate potential biases.MethodsThis European test-negative multicentre study tested primary care patients with acute respiratory symptoms for SARS-CoV-2 in the BA.1/BA.2-dominant period. We estimated PS and booster VE among adults and adolescents (PS only) for all products combined and for Comirnaty alone, by time since vaccination, age and chronic condition. We investigated potential bias due to correlation between COVID-19 and influenza vaccination and explored effect modification and confounding by prior SARS-CoV-2 infection.ResultsAmong adults, PS VE was 37% (95% CI: 24-47%) overall and 60% (95% CI: 44-72%), 43% (95% CI: 26-55%) and 29% (95% CI: 13-43%) < 90, 90-179 and ≥ 180 days post vaccination, respectively. Booster VE was 42% (95% CI: 32-51%) overall and 56% (95% CI: 47-64%), 22% (95% CI: 2-38%) and 3% (95% CI: -78% to 48%), respectively. Primary series VE was similar among adolescents. Restricting analyses to Comirnaty had little impact. Vaccine effectiveness was higher among older adults. There was no signal of bias due to correlation between COVID-19 and influenza vaccination. Confounding by previous infection was low, but sample size precluded definite assessment of effect modification.ConclusionPrimary series and booster VE against symptomatic infection with BA.1/BA.2 ranged from 37% to 42%, with similar waning post vaccination. Comprehensive data on previous SARS-CoV-2 infection would help disentangle vaccine- and infection-induced immunity.

Effectiveness of a fourth SARS-CoV-2 vaccine dose in previously infected individuals from Austria

INTRODUCTION

Evidence is limited on the effectiveness of a fourth vaccine dose against coronavirus disease 2019 (COVID-19) in populations with prior severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections. We estimated the risk of COVID-19 deaths and SARS-CoV-2 infections according to vaccination status in previously infected individuals in Austria.

METHODS

This is a nationwide retrospective observational study. We calculated age and gender adjusted Cox proportional hazard ratios (HRs) of COVID-19 deaths (primary outcome) and SARS-CoV-2 infections (secondary outcome) from 1 November to 31 December 2022, primarily comparing individuals with four versus three vaccine doses. Relative vaccine effectiveness (rVE) was calculated as (1-HR) X 100.

RESULTS

Among 3,986,312 previously infected individuals, 281,291 (7,1%) had four and 1,545,242 (38.8%) had three vaccinations at baseline. We recorded 69 COVID-19 deaths and 89,056 SARS-CoV-2 infections. rVE for four versus three vaccine doses was -24% (95% CI: -120 to 30) against COVID-19 deaths, and 17% (95% CI: 14-19) against SARS-CoV-2 infections. This latter effect rapidly diminished over time and infection risk with four vaccinations was higher compared to less vaccinated individuals during extended follow-up until June 2023. Adjusted HR (95% CI) for all-cause mortality for four versus three vaccinations was 0.79 (0.74-0.85).

DISCUSSION

In previously infected individuals, a fourth vaccination was not associated with COVID-19 death risk, but with transiently reduced risk of SARS-CoV-2 infections and reversal of this effect in longer follow-up. All-cause mortality data suggest healthy vaccinee bias.

The test-negative design: Opportunities, limitations and biases

The purpose of this paper is to investigate how to interpret the outcome of a test-negative design study. We does so by systematically reviewing the properties of the design in relation to its potential applications. We first argue that the application of the design does not depend on certain assumptions (as is sometimes articulated in the literature), which may open new opportunities for using the design. Then, we relate a number of limitations of the design. The design cannot be used for studying the mortality effects of vaccines and is problematic for studies into the effect on hospitalization. The vaccine's effectiveness on the transmission of viruses is also potentially problematic, depending very much on the characteristics of the tests. The implication of our findings is that the test-negative designs can, at best, be seen as an indication of effectiveness in highly idealized situations that are often far away from reality.

COVID-19 vaccine effectiveness against hospitalizations in Paraguay, May 2021-April 2022: A test-negative design

BACKGROUND

Vaccine effectiveness (VE) estimates vary by population characteristics and circulating variants. North America and Europe have generated many COVID-19 VE estimates but relied heavily on mRNA vaccines. Fewer estimates are available for non-mRNA vaccines and from Latin America. We aimed to estimate the effectiveness of several COVID-19 vaccines in preventing SARS-CoV-2-associated severe acute respiratory infection (SARI) in Paraguay from May 2021 to April 2022.

METHODS

Using sentinel surveillance data from four hospitals in Paraguay, we conducted a test-negative case-control study to estimate COVID-19 vaccine effectiveness against SARI by vaccine type/brand and period of SARS-CoV-2 variant predominance (Gamma, Delta, Omicron). We used multivariable logistic regression adjusting for month of symptom onset, age group, and presence of ≥1 comorbidity to estimate the odds of COVID-19 vaccination in SARS-CoV-2 test-positive SARI case-patients compared to SARS-CoV-2 test-negative SARI control-patients.

RESULTS

Of 4,229 SARI patients, 2,381 (56%) were SARS-CoV-2-positive case-patients and 1,848 (44%) were SARS-CoV-2-negative control-patients. A greater proportion of case-patients (73%; 95% CI: 71-75) than of control-patients (40%; 95% CI: 38-42) were unvaccinated. During the Gamma variant-predominant period, VE estimates for partial vaccination with mRNA vaccines and Oxford/AstraZeneca Vaxzevria were 90.4% (95% CI: 66.4-97.6) and 52.2% (95% CI: 25.0-69.0), respectively. During the Delta variant-predominant period, VE estimates for complete vaccination with mRNA vaccines, Oxford/AstraZeneca Vaxzevria, or Gamaleya Sputnik V were 90.4% (95% CI: 74.3-97.3), 83.2% (95% CI: 67.8-91.9), and 82.9% (95% CI: 53.0-95.2), respectively. The effectiveness of all vaccines declined substantially during the Omicron variant-predominant period.

CONCLUSIONS

This study contributes to our understanding of COVID-19 VE in Latin America and to global understanding of vaccines that have not been widely used in North America and Europe. VE estimates from Paraguay can parameterize models to estimate the impact of the national COVID-19 vaccination campaign in Paraguay and similar settings.

The quality and pattern of rehabilitation interventions prescribed for post-COVID-19 infection patients: A systematic review and meta-analysis

Patients with prior COVID-19 infection may present with lasting multisystem symptoms that require intervention and includes exercise rehabilitation. The aim of this systematic review was to investigate the quality of articles, with emphasis on exercise rehabilitation, and conduct a meta-analysis on experimental and observational rehabilitation intervention studies on cardiorespiratory fitness and pulmonary function in post-COVID-19 infection patients. This systematic review used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines for evaluating rehabilitation interventions in clinical practice. A methodical search of cohort and experimental studies occurred from January 2019 up to March 2023. Thirty-two studies were included for complete analysis. The quality of the eligible studies for complete review was fair overall. The studies did not provide a detailed account of key descriptors of exercise such as volume, progression, motivational strategies, adherence and replication. There was a significant difference in the 6-minute walk test (Mean difference (MD) = 51.69 m; confidence intervals (CIs) = 36.99 to 66.38; p < 0.001, level of heterogeneity (I2) = 0), percent of predicted forced expiratory volume in one second (MD = 7.95%; CIs = 3.46 to 12.44; p = 0.0005, I2 = 65%) and percent of predicted forced vital capacity (MD = 3.49%; 95% CI = 1.25 to 5.73; p = 0.002; I2 = 37%) between the experiment and control groups. The current evidence suggests that rehabilitation interventions significantly improve cardiorespiratory fitness and pulmonary function in post-COVID-19 infection patients; however, there is a need for conceptualising high-quality and long-term rehabilitation interventions, especially exercise interventions.

Hierarchical Vaccine Allocation Based on Epidemiological and Behavioral Considerations

Vaccines have proven useful in curbing contagion from new strains of the SARS-CoV-2 virus. However, equitable vaccine allocation continues to be a significant challenge worldwide, necessitating a comprehensive allocation strategy incorporating heterogeneity in epidemiological and behavioral considerations. In this paper, we present a hierarchical allocation strategy that assigns vaccines to zones and their constituent neighborhoods cost-effectively, based on their population density, susceptibility, infected count, and attitude towards vaccinations. Moreover, it includes a module that tackles vaccine shortages in certain zones by locally transferring vaccines from zones with surplus vaccines. We leverage the epidemiological, socio-demographic, and social media datasets from Chicago and Greece and their constituent community areas to show that the proposed allocation approach assigns vaccines based on the chosen criteria and captures the effects of disparate vaccine adoption rates. We conclude the paper with a lowdown on future efforts to extend this study to design models for effective public policies and vaccination strategies that curtail vaccine purchase costs.

Does natural and hybrid immunity obviate the need for frequent vaccine boosters against SARS-CoV-2 in the endemic phase?

The coronavirus disease 2019 (COVID-19) pandemic has entered its endemic phase and we observe significantly declining infection fatality rates due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). On this background, it is crucial but challenging to define current and future vaccine policy in a population with a high immunity against SARS-CoV-2 conferred by previous infections and/or vaccinations. Vaccine policy must consider the magnitude of the risks conferred by new infection(s) with current and evolving SARS-CoV-2 variants, how these risks vary in different groups of individuals, how to balance these risks against the apparently small, but existent, risks of harms of vaccination, and the cost-benefit of different options. More evidence from randomized controlled trials and continuously accumulating national health data is required to inform shared decision-making with people who consider vaccination options. Vaccine policy makers should cautiously weight what vaccination schedules are needed, and refrain from urging frequent vaccine boosters unless supported by sufficient evidence.

Vaccine effectiveness of primary series and booster doses against covid-19 associated hospital admissions in the United States: living test negative design study

OBJECTIVE

To compare the effectiveness of a primary covid-19 vaccine series plus booster doses with a primary series alone for the prevention of hospital admission with omicron related covid-19 in the United States.

DESIGN

Multicenter observational case-control study with a test negative design.

SETTING

Hospitals in 18 US states.

PARTICIPANTS

4760 adults admitted to one of 21 hospitals with acute respiratory symptoms between 26 December 2021 and 30 June 2022, a period when the omicron variant was dominant. Participants included 2385 (50.1%) patients with laboratory confirmed covid-19 (cases) and 2375 (49.9%) patients who tested negative for SARS-CoV-2 (controls).

MAIN OUTCOME MEASURES

The main outcome was vaccine effectiveness against hospital admission with covid-19 for a primary series plus booster doses and a primary series alone by comparing the odds of being vaccinated with each of these regimens versus being unvaccinated among cases versus controls. Vaccine effectiveness analyses were stratified by immunosuppression status (immunocompetent, immunocompromised). The primary analysis evaluated all covid-19 vaccine types combined, and secondary analyses evaluated specific vaccine products.

RESULTS

Overall, median age of participants was 64 years (interquartile range 52-75 years), 994 (20.8%) were immunocompromised, 85 (1.8%) were vaccinated with a primary series plus two boosters, 1367 (28.7%) with a primary series plus one booster, and 1875 (39.3%) with a primary series alone, and 1433 (30.1%) were unvaccinated. Among immunocompetent participants, vaccine effectiveness for prevention of hospital admission with omicron related covid-19 for a primary series plus two boosters was 63% (95% confidence interval 37% to 78%), a primary series plus one booster was 65% (58% to 71%), and for a primary series alone was 37% (25% to 47%) (P<0.001 for the pooled boosted regimens compared with a primary series alone). Vaccine effectiveness was higher for a boosted regimen than for a primary series alone for both mRNA vaccines (BNT162b2 (Pfizer-BioNTech): 73% (44% to 87%) for primary series plus two boosters, 64% (55% to 72%) for primary series plus one booster, and 36% (21% to 48%) for primary series alone (P<0.001); mRNA-1273 (Moderna): 68% (17% to 88%) for primary series plus two boosters, 65% (55% to 73%) for primary series plus one booster, and 41% (25% to 54%) for primary series alone (P=0.001)). Among immunocompromised patients, vaccine effectiveness for a primary series plus one booster was 69% (31% to 86%) and for a primary series alone was 49% (30% to 63%) (P=0.04).

CONCLUSION

During the first six months of 2022 in the US, booster doses of a covid-19 vaccine provided additional benefit beyond a primary vaccine series alone for preventing hospital admissions with omicron related covid-19.

READERS' NOTE

This article is a living test negative design study that will be updated to reflect emerging evidence. Updates may occur for up to two years from the date of original publication.

Estimating conditional vaccine effectiveness

Vaccine effectiveness for COVID-19 is typically estimated for different outcomes that often are hierarchical in severity (e.g. any documented infection, symptomatic infection, hospitalization, death) and subsets of each other. Conditional effectiveness for a more severe outcome conditional on a less severe outcome is the protection offered against the severe outcome (e.g. death) among those who already sustained the less severe outcome (e.g. documented infection). The concept applies also to the protection offered by previous infection rather than vaccination. Formulas and a nomogram are provided here for calculating conditional effectiveness. Illustrative examples are presented from recent vaccine effectiveness studies, including situations where effectiveness for different outcomes changed at different pace over time. E(death | documented infection) is the percent decrease in the case fatality rate and E(death | infection) is the percent decrease in the infection fatality rate (IFR). Conditional effectiveness depends on many factors and should not be misinterpreted as a causal effect estimate. However, it may be used for better personalized communication of the benefits of vaccination, considering also IFR and epidemic activity in public health decision-making and communication.

Risk and severity of SARS-CoV-2 reinfections during 2020–2022 in Vojvodina, Serbia: A population-level observational study

Background

Data on the rate and severity of SARS-CoV-2 reinfections in real-world settings are scarce and the effects of vaccine boosters on reinfection risk are unknown.

Methods

In a population-level observational study, registered SARS-CoV-2 laboratory-confirmed Vojvodina residents, between March 6, 2020 and October 31, 2021, were followed for reinfection ≥90 days after primary infection. Data were censored at the end of follow-up (January 31, 2022) or death. The reinfection risk was visualized with Kaplan-Meier plots. To examine the protective effect of vaccination, the subset of individuals with primary infection in 2020 (March 6–December 31) were matched (1:2) with controls without reinfection.

Findings

Until January 31, 2022, 13,792 reinfections were recorded among 251,104 COVID-19 primary infections (5.49%). Most reinfections (86.77%, 11,967/13,792) were recorded in January 2022. Reinfections were mostly mild (99.17%, 13,678/13,792). Hospitalizations were uncommon [1.08% (149/13,792) vs. 3.66% (505/13,792) in primary infection] and COVID-19 deaths were very rare (20/13,792, case fatality rate 0.15%). The overall incidence rate of reinfections was 5.99 (95% CI 5.89–6.09) per 1000 person-months. The reinfection risk was estimated as 0.76% at six months, 1.36% at nine months, 4.96% at 12 months, 16.68% at 15 months, and 18.86% at 18 months. Unvaccinated (OR=1.23; 95%CI=1.14–1.33), incompletely (OR=1.33; 95%CI=1.08–1.64) or completely vaccinated (OR=1.50; 95%CI=1.37–1.63), were modestly more likely to be reinfected compared with recipients of a third (booster) vaccine dose.

Interpretation

SARS-CoV-2 reinfections were uncommon until the end of 2021 but became common with the advent of Omicron. Very few reinfections were severe. Boosters may modestly reduce reinfection risk.

Funding

No specific funding was obtained for this study.

COVID-19 vaccination effectiveness rates by week and sources of bias: a retrospective cohort study

OBJECTIVE

To examine COVID-19 vaccine effectiveness over six 7-day intervals after the first dose and assess underlying bias in observational data.

DESIGN AND SETTING

Retrospective cohort study using Columbia University Irving Medical Center data linked to state and city immunisation registries.

OUTCOMES AND MEASURES

We used large-scale propensity score matching with up to 54 987 covariates, fitted Cox proportional hazards models and constructed Kaplan-Meier plots for two main outcomes (COVID-19 infection and COVID-19-associated hospitalisation). We conducted manual chart review of cases in week 1 in both groups along with a set of secondary analyses for other index date, outcome and population choices.

RESULTS

The study included 179 666 patients. We observed increasing effectiveness after the first dose of mRNA vaccines with week 6 effectiveness approximating 84% (95% CI 72% to 91%) for COVID-19 infection and 86% (95% CI 69% to 95%) for COVID-19-associated hospitalisation. When analysing unexpectedly high effectiveness in week 1, chart review revealed that vaccinated patients are less likely to seek care after vaccination and are more likely to be diagnosed with COVID-19 during the encounters for other conditions. Secondary analyses highlighted potential outcome misclassification for International Classification of Diseases, Tenth Revision, Clinical Modification diagnosis, the influence of excluding patients with prior COVID-19 infection and anchoring in the unexposed group. Long-term vaccine effectiveness in fully vaccinated patients matched the results of the randomised trials.

CONCLUSIONS

For vaccine effectiveness studies, observational data need to be scrutinised to ensure compared groups exhibit similar health-seeking behaviour and are equally likely to be captured in the data. While we found that studies may be capable of accurately estimating long-term effectiveness despite bias in early weeks, the early week results should be reported in every study so that we may gain a better understanding of the biases. Given the difference in temporal trends of vaccine exposure and patients' baseline characteristics, indirect comparison of vaccines may produce biased results.