The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has claimed millions of lives worldwide. Mass vaccination is considered one of the most important strategies to prevent morbidity, mortality, and transmission of the virus. Clinical trials and real-world effectiveness studies have shown COVID-19 vaccines to be effective against SARS-CoV-2 infections. However, most of the studies do not assess the effectiveness of the vaccine against asymptomatic infection.
Study: Covid-19 Vaccine Effectiveness in Healthcare Personnel in six Israeli Hospitals (CoVEHPI). Image Credit: Cryptographer/ Shutterstock
Studies showed that COVID-19 vaccines helped to reduce asymptomatic infection as well as onward viral transmission. Vaccine effectiveness (VE) studies from the US and UK showed high two-dose VE with the Pfizer BNT162b2 mRNA vaccine. However, the UK studies collected PCR samples from patients with asymptomatic infections biweekly and might have missed asymptomatic infections that caused viral shredding only for a few days. The studies did not include any serological tests that could detect new infections missed by respiratory swab tests.
Furthermore, studies from the US and Israel showed that the viral RNA load is much lower in infected vaccinated individuals than in unvaccinated individuals. Also, studies from England and Scotland showed that secondary infections were much lower among families of vaccinated individuals than unvaccinated families.
A study published on preprint server medRxiv* aimed to evaluate the VE against SARS-CoV-2 infection by carrying out a prospective cohort study that included healthcare personnel (HCP) in six hospitals in Israel.
A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.
About the study
HCP working at six designated hospitals and eligible to receive the COVID-19 vaccine were enrolled for this study. The enrolment started on December 27, 2020, and continued through February 15, 2021.
Respiratory and serological specimens were collected from the participants at enrolment. They were also asked to fill out a questionnaire about demographic information, community, attitude about the COVID-19 vaccine, and symptoms in the last week. Participants had to fill out an electronic symptom questionnaire every 12 weeks. Additionally, weekly nasal or nasal and throat samples were collected even if the participants did not show any symptoms.
Blood samples for serology were collected from participants at 30 days and 90 days after enrolment for both unvaccinated individuals and individuals who had received the first dose of the Pfizer BNT162b2 mRNA vaccine. Data on the patient’s age, sex, population sector, history of influenza vaccination in the last five years, and history of COVID-19 vaccination was extracted, along with data on chronic medical conditions and risk factors. Data on PCR tests before and during the study was also collected, along with hospitalization data of patients who tested positive for SARS-CoV-2 during the study period.
The respiratory specimens were tested by carrying out real-time polymerase chain reaction reverse transcription (RT-PCR) tests. The PCR positive samples then underwent genetic analysis studies, and the serology samples were tested by determining the presence of antibodies against SARS-CoV-2.
The primary analysis was performed by evaluating VE against PCR confirmed COVID-19 seven days after receiving the second dose of the vaccine compared to unvaccinated individuals. Secondary analysis involved estimation of VE against symptomatic and asymptomatic PCR confirmed COVID-19 separately and VE fourteen days after the second dose of the vaccine.
Finally, follow-up time was calculated in person-days (PD) from the day of enrolment. The study involved the follow-up of participants 90 days from the enrolment. Although, the day of enrolment varies depending on whether the participants were unvaccinated or previously vaccinated.
Of the 1567 HCP participants enrolled in the study, 1250 were included in the primary analysis, and 222 were excluded due to positive PCR or serology tests before or at enrolment. Out of the 1250 participants, 998 were vaccinated at or before enrolment, and 252 were unvaccinated at enrolment.
Most of the vaccinated and unvaccinated individuals were female. The median age of vaccinated individuals was 47 years as compared to 37 years of unvaccinated individuals. More physicians were included in vaccinated individuals as compared to unvaccinated individuals.
For the primary analysis, 68574 PDs were contributed by vaccinated individuals, and unvaccinated individuals contributed 10027 PDs. Vaccinated individuals were followed up for 71, whereas unvaccinated individuals were followed up for 35 days.
During the primary analysis, 13 participants tested PCR positive, out of which four of them were vaccinated, and the VE was about 94.5 percent. Eleven participants out of the 13 participants were asymptomatic, and two were symptomatic. However, the symptomatic individuals did not require hospitalization. It was also noted that the results of the secondary analyses were very similar to the primary analysis.
Sequence analysis of the PCR-positive cases indicated that the SARS-CoV-2 variant causing infection was the alpha variant which was thought to predominate in the country during the study period.
The researchers concluded that their results suggest that two doses of the Pfizer BNT162b2 mRNA vaccine against SARS-CoV-2, during a period of predominant alpha variant circulation, result in high VE.
The study had several limitations. Firstly, because the population was voluntary, the study may suffer from selection bias. Secondly, many demographic differences existed among vaccinated and unvaccinated individuals. Thirdly, serology tests were combined in the study whose sensitivity might be variable, creating inconsistencies. Finally, the serological tests were not very effective in capturing asymptomatic cases.
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.