Effectiveness of COVID-19 vaccines over time prior to Omicron emergence in Ontario, Canada: test-negative design study

Bias in vaccine effectiveness studies of clinically severe outcomes that are measured with low specificity: the example of COVID-19-related hospitalisation

Many vaccine effectiveness (VE) analyses of severe disease outcomes such as hospitalisation and death include 'false' cases that are not actually caused by the infection or disease under study. While the inclusion of such false cases inflate outcome rates in both vaccinated and unvaccinated populations, it is less obvious how they affect estimates of VE. Illustrating the main points through simple examples, this article shows how VE is underestimated when false cases are included as outcomes. Depending how the outcome indicator is defined, estimates of VE against severe disease outcomes, whose definition allows for the inclusion of false cases, will be biased downwards and may in certain circumstances approximate the same level as the VE against infection. The bias is particularly pronounced for vaccines that offer high levels of protection against severe disease outcomes but poor protection against infection. Analysing outcomes that are measured with low sensitivity generally does not cause bias in VE studies; defining outcome indicators that minimise the number of false cases rather than the number of missed cases is preferable in VE studies.

Effectiveness of a Fourth COVID-19 mRNA Vaccine Dose Against the Omicron Variant in Solid Organ Transplant Recipients

BACKGROUND

The effectiveness of booster doses of COVID-19 vaccines in solid organ transplant recipients is unclear. We conducted a population-based matched cohort study using linked administrative healthcare databases from Ontario, Canada to estimate the marginal vaccine effectiveness of a fourth versus third dose of the BNT162b2 and mRNA-1273 vaccines against clinically important outcomes (ie, hospitalization or death) and infection during the era of the Omicron variant.

METHODS

We matched 3120 solid organ transplant recipients with a third COVID-19 vaccine dose (reference) to 3120 recipients with a fourth dose. Recipients were matched on the third dose date (±7 d). We used a multivariable Cox proportional hazards model to estimate the marginal vaccine effectiveness with outcomes occurring between December 21, 2021 and April 30, 2022.

RESULTS

The cumulative incidence of COVID-19-related hospitalization or death was 2.8% (95% confidence interval [CI], 2.0-3.7) in the third dose group compared with 1.1% (95% CI, 0.59-1.8) in the fourth dose group after 84 d of follow-up (P < 0.001). The adjusted marginal vaccine effectiveness was 70% (95% CI, 47-83) against clinically important outcomes and 39% (95% CI, 21-52) against SARS-CoV-2 infection.

CONCLUSIONS

Compared with a third dose, a fourth dose of the COVID-19 vaccine was associated with improved protection against hospitalization, death, and SARS-CoV-2 infection during the Omicron era. Results highlight the importance of a booster COVID-19 vaccine dose in solid organ transplant recipients.

Comparison of Medical and Mental Health Sequelae Following Hospitalization for COVID-19, Influenza, and Sepsis

Importance

People who survive hospitalization for COVID-19 are at risk for developing new cardiovascular, neurological, mental health, and inflammatory autoimmune conditions. It is unclear how posthospitalization risks for COVID-19 compare with those for other serious infectious illnesses.

Objective

To compare risks of incident cardiovascular, neurological, and mental health conditions and rheumatoid arthritis in 1 year following COVID-19 hospitalization against 3 comparator groups: prepandemic hospitalization for influenza and hospitalization for sepsis before and during the COVID-19 pandemic.

Design, Setting, and Participants

This population-based cohort study included all adults hospitalized for COVID-19 between April 1, 2020, and October 31, 2021, historical comparator groups of people hospitalized for influenza or sepsis, and a contemporary comparator group of people hospitalized for sepsis in Ontario, Canada.

Exposure

Hospitalization for COVID-19, influenza, or sepsis.

Main Outcome and Measures

New occurrence of 13 prespecified conditions, including cardiovascular, neurological, and mental health conditions and rheumatoid arthritis, within 1 year of hospitalization.

Results

Of 379 366 included adults (median [IQR] age, 75 [63-85] years; 54% female), there were 26 499 people who survived hospitalization for COVID-19, 299 989 historical controls (17 516 for influenza and 282 473 for sepsis), and 52 878 contemporary controls hospitalized for sepsis. Hospitalization for COVID-19 was associated with an increased 1-year risk of venous thromboembolic disease compared with influenza (adjusted hazard ratio, 1.77; 95% CI, 1.36-2.31) but with no increased risks of developing selected ischemic and nonischemic cerebrovascular and cardiovascular disorders, neurological disorders, rheumatoid arthritis, or mental health conditions compared with influenza or sepsis cohorts.

Conclusions and Relevance

In this cohort study, apart from an elevated risk of venous thromboembolism within 1 year, the burden of postacute medical and mental health conditions among those who survived hospitalization for COVID-19 was comparable with other acute infectious illnesses. This suggests that many of the postacute consequences of COVID-19 may be related to the severity of infectious illness necessitating hospitalization rather than being direct consequences of infection with SARS-CoV-2.

Evaluation of Waning of SARS-CoV-2 Vaccine-Induced Immunity: A Systematic Review and Meta-analysis

Importance

Estimates of the rate of waning of vaccine effectiveness (VE) against COVID-19 are key to assess population levels of protection and future needs for booster doses to face the resurgence of epidemic waves.

Objective

To quantify the progressive waning of VE associated with the Delta and Omicron variants of SARS-CoV-2 by number of received doses.

Data Sources

PubMed and Web of Science were searched from the databases' inception to October 19, 2022, as well as reference lists of eligible articles. Preprints were included.

Study Selection

Selected studies for this systematic review and meta-analysis were original articles reporting estimates of VE over time against laboratory-confirmed SARS-CoV-2 infection and symptomatic disease.

Data Extraction and Synthesis

Estimates of VE at different time points from vaccination were retrieved from original studies. A secondary data analysis was performed to project VE at any time from last dose administration, improving the comparability across different studies and between the 2 considered variants. Pooled estimates were obtained from random-effects meta-analysis.

Main Outcomes and Measures

Outcomes were VE against laboratory-confirmed Omicron or Delta infection and symptomatic disease and half-life and waning rate associated with vaccine-induced protection.

Results

A total of 799 original articles and 149 reviews published in peer-reviewed journals and 35 preprints were identified. Of these, 40 studies were included in the analysis. Pooled estimates of VE of a primary vaccination cycle against laboratory-confirmed Omicron infection and symptomatic disease were both lower than 20% at 6 months from last dose administration. Booster doses restored VE to levels comparable to those acquired soon after the administration of the primary cycle. However, 9 months after booster administration, VE against Omicron was lower than 30% against laboratory-confirmed infection and symptomatic disease. The half-life of VE against symptomatic infection was estimated to be 87 days (95% CI, 67-129 days) for Omicron compared with 316 days (95% CI, 240-470 days) for Delta. Similar waning rates of VE were found for different age segments of the population.

Conclusions and Relevance

These findings suggest that the effectiveness of COVID-19 vaccines against laboratory-confirmed Omicron or Delta infection and symptomatic disease rapidly wanes over time after the primary vaccination cycle and booster dose. These results can inform the design of appropriate targets and timing for future vaccination programs.

Methodological changes implemented over time to support accurate and timely COVID-19 vaccine coverage estimates: Ontario, Canada

The COVID-19 vaccination program implementation in Ontario, Canada has spanned multiple years and is ongoing. To meet the challenges of the program, Ontario developed and implemented a new electronic COVID-19 immunization registry, COVaxON, which captures individual-level data on all doses administered in the province enabling comprehensive coverage assessment. However, the need for ongoing COVID-19 vaccine coverage assessments over a multi-year vaccination program posed challenges necessitating methodological changes. This paper describes Ontario's COVID-19 immunization registry, the methods implemented over time to allow for the ongoing assessment of vaccine coverage by age, and the impact of those methodological changes. Throughout the course of the vaccination program, four different methodological approaches were used to calculate age-specific coverage estimates using vaccination data (numerator) obtained from COVaxON. Age-specific numerators were initially calculated using age at time of first dose (method A), but were updated to the age at coverage assessment (method B). Database enhancements allowed for the exclusion of deceased individuals from the numerator (method C). Population data (denominator) was updated to 2022 projections from the 2021 national census following their availability (method D). The impact was most evident in older age groups where vaccine uptake was high. For example, coverage estimates for individuals aged 70-79 years of age for at least one dose decreased from 104.9 % (method B) to 95.0 % (method D). Thus, methodological changes improved estimates such that none exceeded 100 %. Ontario's COVID-19 immunization registry has been transformational for vaccine program surveillance. The implementation of a single registry for COVID-19 vaccines was essential for comprehensive near real-time coverage assessment, and enabled new uses of the data to support additional components of vaccine program surveillance. The province is well positioned to build on what has been achieved as a result of the COVID-19 pandemic and expand the registry to other routine vaccination programs.

Population-based evaluation of the effectiveness of nirmatrelvir-ritonavir for reducing hospital admissions and mortality from COVID-19

BACKGROUND

A randomized controlled trial involving a high-risk, unvaccinated population that was conducted before the Omicron variant emerged found that nirmatrelvir-ritonavir was effective in preventing progression to severe COVID-19. Our objective was to evaluate the effectiveness of nirmatrelvir-ritonavir in preventing severe COVID-19 while Omicron and its subvariants predominate.

METHODS

We conducted a population-based cohort study in Ontario that included all residents who were older than 17 years of age and had a positive polymerase chain reaction test for SARS-CoV-2 between Apr. 4 and Aug. 31, 2022. We compared patients treated with nirmatrelvir-ritonavir with patients who were not treated and measured the primary outcome of hospital admission from COVID-19 or all-cause death at 1-30 days, and a secondary outcome of all-cause death. We used weighted logistic regression to calculate weighted odds ratios (ORs) with confidence intervals (CIs) using inverse probability of treatment weighting (IPTW) to control for confounding.

RESULTS

The final cohort included 177 545 patients, 8876 (5.0%) who were treated with nirmatrelvir-ritonavir and 168 669 (95.0%) who were not treated. The groups were well balanced with respect to demographic and clinical characteristics after applying stabilized IPTW. We found that the occurrence of hospital admission or death was lower in the group given nirmatrelvir-ritonavir than in those who were not (2.1% v. 3.7%; weighted OR 0.56, 95% CI 0.47-0.67). For death alone, the weighted OR was 0.49 (95% CI 0.39-0.62). Our findings were similar across strata of age, drug-drug interactions, vaccination status and comorbidities. The number needed to treat to prevent 1 case of severe COVID-19 was 62 (95% CI 43-80), which varied across strata.

INTERPRETATION

Nirmatrelvir-ritonavir was associated with significantly reduced odds of hospital admission and death from COVID-19, which supports use to treat patients with mild COVID-19 who are at risk for severe disease.

Long-term effectiveness of COVID-19 vaccines against infections, hospitalisations, and mortality in adults: findings from a rapid living systematic evidence synthesis and meta-analysis up to December, 2022

BACKGROUND

Synthesising evidence on the long-term vaccine effectiveness of COVID-19 vaccines (BNT162b2 [Pfizer-BioNTech], mRNA-1273 [Moderna], ChAdOx1 nCoV-19 [AZD1222; Oxford-AstraZeneca], and Ad26.COV2.S [Janssen]) against infections, hospitalisations, and mortality is crucial to making evidence-based pandemic policy decisions.

METHODS

In this rapid living systematic evidence synthesis and meta-analysis, we searched EMBASE and the US National Institutes of Health's iSearch COVID-19 Portfolio, supplemented by manual searches of COVID-19-specific sources, until Dec 1, 2022, for studies that reported vaccine effectiveness immediately and at least 112 days after a primary vaccine series or at least 84 days after a booster dose. Single reviewers assessed titles, abstracts, and full-text articles, and extracted data, with a second reviewer verifying included studies. The primary outcomes were vaccine effectiveness against SARS-CoV-2 infections, hospitalisations, and mortality, which were assessed using three-level meta-analytic models. This study is registered with the National Collaborating Centre for Methods and Tools, review 473.

FINDINGS

We screened 16 696 records at the title and abstract level, appraised 832 (5·0%) full texts, and initially included 73 (0·4%) studies. Of these, we excluded five (7%) studies because of critical risk of bias, leaving 68 (93%) studies that were extracted for analysis. For infections caused by any SARS-CoV-2 strain, vaccine effectiveness for the primary series reduced from 83% (95% CI 80-86) at baseline (14-42 days) to 62% (53-69) by 112-139 days. Vaccine effectiveness at baseline was 92% (88-94) for hospitalisations and 91% (85-95) for mortality, and reduced to 79% (65-87) at 224-251 days for hospitalisations and 86% (73-93) at 168-195 days for mortality. Estimated vaccine effectiveness was lower for the omicron variant for infections, hospitalisations, and mortality at baseline compared with that of other variants, but subsequent reductions occurred at a similar rate across variants. For booster doses, which covered mostly omicron studies, vaccine effectiveness at baseline was 70% (56-80) against infections and 89% (82-93) against hospitalisations, and reduced to 43% (14-62) against infections and 71% (51-83) against hospitalisations at 112 days or later. Not enough studies were available to report on booster vaccine effectiveness against mortality.

INTERPRETATION

Our analyses indicate that vaccine effectiveness generally decreases over time against SARS-CoV-2 infections, hospitalisations, and mortality. The baseline vaccine effectiveness levels for the omicron variant were notably lower than for other variants. Therefore, other preventive measures (eg, face-mask wearing and physical distancing) might be necessary to manage the pandemic in the long term.

FUNDING

Canadian Institutes of Health Research and the Public Health Agency of Canada.