The aim of this study is to find out whether healthcare workers who have evidence of prior COVID-19, detected by positive antibody tests, compared to those who do not have evidence of infection (negative antibody tests) are protected from future episodes of infection. We will recruit healthcare workers to be followed for at least a year and study their immune response to the virus causing COVID-19.
We will do this by collecting data on their history of COVID-19 infection and any new symptoms. Individuals who work at a healthcare site will be asked to have a swab to detect the virus causing COVID-19 every other week in order to detect mild cases or cases without symptoms.
This is the main test that is currently used to detect and diagnose infection. It looks directly for the virus in the nose and/ or throat. Once the infection is cleared, we cannot detect virus in swab samples.
Therefore, we will also ask these individuals to have blood samples taken at least once per month to determine whether they have antibodies to the infection. These blood samples allow the previous infection to be detected because the response to infection (the immune response) in the body is to produce antibodies in the blood. It takes up to 4 weeks to make enough antibodies to fight the infection.
But once someone recovers, antibodies stay in the blood at low levels– this is may help prevent re-infection. For COVID-19 we do not know yet if the detection of antibodies protects people from future infections. Through this study, we will provide this very important information which will help us to understand the future impact of COVID-19 on the population
23,324 participants from 104 sites (all in England) met the inclusion criteria for this analysis and were enrolled. Included participants had a median age of 46·1 years (IQR 36·0–54·1) and 19 692 (84%) were female; 8203 (35%) were assigned to the positive cohort at the start of the analysis period, and 15,121 (65%) assigned to the negative cohort.
Total follow-up time was 2 calendar months and 1,106,905 person-days (396,318 vaccinated and 710,587 unvaccinated). Vaccine coverage was 89% on Feb 5, 2021, 94% of whom had BNT162b2 vaccine.
Significantly lower coverage was associated with previous infection, gender, age, ethnicity, job role, and Index of Multiple Deprivation score. During follow-up, there were 977 new infections in the unvaccinated cohort, an incidence density of 14 infections per 10,000 person-days; the vaccinated cohort had 71 new infections 21 days or more after their first dose (incidence density of eight infections per 10,000 person-days) and nine infections 7 days after the second dose (incidence density four infections per 10,000 person-days).
In the unvaccinated cohort, 543 (56%) participants had typical COVID-19 symptoms and 140 (14%) were asymptomatic on or 14 days before their PCR positive test date, compared with 29 (36%) with typical COVID-19 symptoms and 15 (19%) asymptomatic in the vaccinated cohort. A single dose of BNT162b2 vaccine showed vaccine effectiveness of 70% (95% CI 55–85) 21 days after first dose and 85% (74–96) 7 days after two doses in the study population.
The Lancet findings show that the BNT162b2 vaccine can prevent both symptomatic and asymptomatic infection in working-age adults. This cohort was vaccinated when the dominant variant in circulation was B1.1.7 and shows effectiveness against this variant.