Systematic review finds COVID-19 vaccine efficacy remains high for at least six months

A team of international scientists has recently conducted a systematic review of published studies investigating the duration of efficacy of coronavirus disease 2019 (COVID-19) vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and disease severity. The findings reveal that the vaccine efficacy against severe COVID-19 remains high for six months after full vaccination. The study is currently available on the preprint server of The Lancet journal.

Study: Duration of Effectiveness of Vaccines Against SARS-CoV-2 Infection and COVID-19 Disease: Results of a Systematic Review and Meta-Regression. Image Credit: Marco Lazzarini/ ShutterstockStudy: Duration of Effectiveness of Vaccines Against SARS-CoV-2 Infection and COVID-19 Disease: Results of a Systematic Review and Meta-Regression. Image Credit: Marco Lazzarini/ Shutterstock

Background

With the progression of the COVID-19 pandemic, several vaccines have been developed and rigorously tested in clinical trials. The vaccines with satisfactory safety and efficacy status have received emergency use approval from country-specific public health authorities and the World Health Organization (WHO). The initial evaluation of vaccine efficacy within two to three months after full vaccination (immunization with two vaccine doses at a fixed interval) has shown that many vaccines have more than 85% efficacy in preventing SARS-CoV-2 infection and symptomatic COVID-19.

However, the recent emergence of vaccine breakthrough cases in many countries across the globe has highlighted the possibility of waning vaccine efficacy with time. Thus, continuous monitoring of vaccine efficacy is highly required to inform public health authorities about the need for a third booster dose.

In the current study, the scientists have assessed the duration of vaccine efficacy against SARS-CoV-2 infection and disease severity by systematically reviewing available literature.

Study design

The analysis included peer-reviewed and preprint studies that were published between June and October 2021. Both randomized controlled trials and observational studies that investigated COVID-19 vaccine efficacy were analyzed in the current study. Specifically, studies comparing fully vaccinated and unvaccinated persons were included in the analysis.

A separate set of analyzes were also conducted to determine the rate of breakthrough infections in people vaccinated at different time points. The studies that identified cases during the SARS-CoV-2 delta outbreak in the population were only included in this analysis.

Important observations

A total of 9,261 studies were initially screened. Of these studies, 14 were finally analyzed for evaluating the duration of vaccine efficacy. The vaccines evaluated in these studies were BNT162b2 developed by Pfizer/BioNTech, mRNA-1273 developed by Moderna, Ad26.COV2.S developed by Janssen/Johnson & Johnson, and AZD1222 developed by Oxford/AstraZeneca.

Vaccine efficacy against SARS-CoV-2 infection

A total of nine studies were identified to have vaccine-specific efficacy data over a six-month period after full vaccination. These studies analyzed vaccine efficacy against any SARS-CoV-2 infections. About 89% of these studies showed a drop in vaccine efficacy by ten or more percentage points from the peak efficacy. A 25-percentage point drop in vaccine efficacy from the peak was observed in 22% of studies investigating the Pfizer vaccine efficacy against different viral variants.

Considering all vaccine efficacy data irrespective of variant types, an 18-percentage point drop in vaccine efficacy against SARS-CoV-2 infection was observed between 1- and 6-months post-vaccination. Considering only the older population, the drop was almost 20 percentage points.

Vaccine efficacy against symptomatic COVID-19

In one study involving the Moderna vaccine, no drop in efficacy against symptomatic COVID-19 was observed for five months. In another study involving the Pfizer and AstraZeneca vaccines, a 10-percentage point drop from the peak value was observed for both vaccines.

Considering all vaccine efficacy data irrespective of variant types, a 25-percentage point drop in vaccine efficacy against symptomatic COVID-19 was observed between one and six months post-vaccination. Considering only the older population, the drop was 32 percentage points.

Vaccine efficacy against severe COVID-19

Four studies were identified to have vaccine efficacy data against severe COVID-19 caused by the delta or alpha variant. None of the vaccines evaluated in these studies showed a drop in efficacy below 70%.

Considering all vaccine efficacy data irrespective of variant types, an 8-percentage point drop in vaccine efficacy against severe COVID-19 was observed between 1- and 6-months post-vaccination. Considering only the older population, the drop was 9.7 percentage points.

Vaccine efficacy against delta infections

In studies involving delta breakthrough infections, a 1.7-times higher rate of infection was observed in early recipients of the Pfizer vaccine compared to recent recipients. In recipients of the Moderna vaccine, the infection rate increased by 1.5-times.  

Regarding disease severity among people aged 60 years or above, a 3-times higher rate of severe breakthrough infection and 1.6-times higher rate of breakthrough hospitalized infection were observed among recipients of the Pfizer vaccine.

Study significance

The study reveals that the efficacy of COVID-19 vaccines against severe disease remained above 70% for at least six months post-full vaccination. However, the efficacy against SARS-CoV-2 infection and symptomatic COVID-19 reduces gradually with time.

*Important notice

Preprints are not Lancet publications or necessarily under review with a Lancet journal. These preprints are early stage research papers that have not been peer-reviewed. The findings should not be used for clinical or public health decision making and should not be presented to a lay audience without highlighting that they are preliminary and have not been peer-reviewed.

Journal reference:
Dr. Sanchari Sinha Dutta

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta is a science communicator who believes in spreading the power of science in every corner of the world. She has a Bachelor of Science (B.Sc.) degree and a Master's of Science (M.Sc.) in biology and human physiology. Following her Master's degree, Sanchari went on to study a Ph.D. in human physiology. She has authored more than 10 original research articles, all of which have been published in world renowned international journals.

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