Protective effect conferred by prior infection and vaccination on COVID-19 in a healthcare worker cohort in South India

The emergence of newer variants with the immune escape potential raises
concerns about breakthroughs and re-infections resulting in future waves
of infection. We examined the protective effect of prior COVID-19 disease
and vaccination on infection rates among a cohort of healthcare workers
(HCW) in South India during the second wave driven mainly by the delta
variant. Symptomatic HCWs were routinely tested by RT-PCR as per
institutional policy. Vaccination was offered to all HCWs in late January,
and the details were documented. We set up a non-concurrent cohort to
document infection rates and estimated protective efficacy of prior
infection and vaccination between 16th Apr to 31st May 2021, using a Cox
proportional hazards model with time-varying covariates adjusting for
daily incidence. Between June 2020 and May 2021, 2735 (23.9%) of 11,405
HCWs were infected, with 1412, including 32 re-infections, reported during
the second wave. 6863 HCWs received two doses of vaccine and 1905 one
dose. The protective efficacy of prior infection against symptomatic
infection was 86.0% (95% CI 76.7% - 91.6%). Vaccination combined with
prior infection provided 91.1% (95% CI 84.1% - 94.9%) efficacy. In the
absence of prior infection, vaccine efficacy against symptomatic infection
during the second wave was 31.8% (95% CI 23.5% – 39.1%). Prior infection
provided substantial protection against symptomatic re-infection and
severe disease during a delta variant-driven second wave in a cohort of
health care workers.