To contain the ongoing coronavirus disease 2019 (COVID-19) pandemic, scientists have worked extensively to develop various therapeutics, vaccines, and many other effective tactics. Many believe that among these strategies, the development of effective COVID-19 vaccines and rapid vaccination of the majority of the world’s population is the only effective means to manage the COVID-19 pandemic. At present, several vaccines have received emergency use authorization (EUA) from several regulatory bodies, and many of the countries around the world have already commenced vaccination programs.
One of the vaccines that have received an EUA is BNT162b2, developed by Pfizer-BioNTech. This mRNA-based vaccine has shown significant effectiveness, in the randomized placebo-controlled Phase III clinical trials, against the symptomatic infection caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the virus behind the COVID-19 pandemic. After the Phase III clinical trials, researchers have estimated the efficacy of this vaccine to be 80% against both asymptomatic and symptomatic infection, 14 days after the first dose and 90% after administration of two doses of the vaccine.
A new study
A new study released onto the medRxiv* preprint server has estimated the effectiveness of vaccines in preventing COVID-19 as a function of time since vaccination. This study included candidates from the USAF Academy, which is a military base in Colorado Springs, CO. This academy consists of around 4,200 cadets between the ages of 17 and 26. During the study period, these military students normally lived in two large dormitories on campus and carried on with their daily training and academic routines. The cadets received the Pfizer-BioNTech vaccine, an FDA-approved vaccine, to protect them from COVID-19.
The vaccinated candidates were tested weekly using the Sofia SARS Antigen Fluorescent Immunoassay (FIA) throughout the study period (from March 1 to May 1, 2021). The authors of this study considered this time period because infection prevalence was greater than 0.8% before March 1, 2021, which subsequently reduced to 0.1% after May 1, 2021. Thereby, this time period represented a significant change in the exposure risk.
The weekly analysis helped to promptly assess the onset of an infection. This surveillance testing helped identify the asymptomatic individuals, whereas symptomatic individuals were identified in the cadet clinic, where they were subjected to RT-PCR tests to confirm COVID-19. Thus, the present study conducts natural experiments to study the development of post-vaccination immune responses among young and healthy individuals.
The main findings
During the study period, the researchers observed that only 3% of the cadets received at least the first dose of the BNT162b2 vaccine in March 2021, which gradually increased to 85% by the end of the study period (May 2021). Further, during this study period, 71% of the cadets received both doses of the vaccine. The Centers for Disease Control (CDC) has described fully vaccinated individuals as those who passed a period of 14 days after receiving the second dose of the vaccine. The researchers reported that 36% of the cadets were fully vaccinated. Based on the weekly surveillance testing of the study group, scientists calculated that the prevalence of COVID-19 infection in the cadet population was constant at about 0.42%.
During the study period, a total of 151 COVID-19 cases had been observed. Among these, 112 individuals were unvaccinated, 37 candidates were partially vaccinated, and only 2 candidates were fully vaccinated. The researchers reported a statistically modest reduction of 1.9 in infection risk after the second-week post-first dose of the mRNA vaccine.
Further, by the start of the third week, a statistically significant four-fold reduction of infection risk was reported. In the fully vaccinated individuals, a statistically significant eleven-fold decrease in the risk of infection was observed. The authors of this study have revealed that the results of the current study are in line with the previous studies on vaccine effectiveness that included healthcare workers in the UK and the USA.
The current research has revealed that in real-world conditions, the BNT162b2 vaccine has shown 90% effectiveness in preventing symptomatic and asymptomatic infections in young and healthy adults. The risk of infection already showed a declining pattern after the second week following a first dose of the vaccine.
One of the limitations of the present study is the variations in SARS-CoV-2 exposure risk during the study period. Another limitation arises from the use of Sofia SARS Antigen Fluorescent Immunoassay for the weekly surveillance of the cadets. This diagnostic tool has been reported as less sensitive for the detection of COVID-19. Future research could focus on improving along these dimensions.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.