Among the participants without previous SARS-CoV-2 infection, two doses of Pfizer vaccine administered with a long interval between doses was associated with a decrease in the risk of infection of 85% (i.e., the adjusted vaccine effectiveness in the first 2 months after the development of the full immune response, 14 to 73 days after the second dose). Over time, the adjusted vaccine effectiveness declined but remained high, at 68%, 134 to 193 days after the second dose. At a median of 201 days (interquartile range, 197 to 205) after the second dose, study observed evidence of waning of protection, with an adjusted vaccine effectiveness of 51%.

A similar trend was observed in the participants who received a second dose of Pfizer vaccine with a short interval between doses, with high protection at 14 to 73 days (adjusted vaccine effectiveness, 89%; that decreased to 53% at a median of 238 days after the second dose. Study found no significant difference between the Pfizer vaccine participants who had a long interval and those who had a short interval between doses with respect to protection after the second dose.

The adjusted effectiveness of two doses of the AstraZeneca vaccine was 58% 14 to 73 days after the second dose. The effectiveness did not differ considerably with longer periods of time after the second dose, with overlapping confidence intervals of vaccine effectiveness reflecting the small number of participants with data used to calculate this estimate.

At 14 to 73 days after the second dose, the BNT162b2 vaccine with a short interval between doses was 74% more effective and the BNT162b2 vaccine with a long interval between doses was 65% more effective than the ChAdOx1 nCoV-19 vaccine.

A total of 6169 participants in the previously infected cohort were followed in the unvaccinated follow-up period and up to 1 year after a primary infection. These participants were predominantly infected in the spring of 2020 and were followed in the period before emergence of the delta (B.1.617.2) variant. The risk of reinfection among these participants was 86% lower than the risk of primary infection among the unvaccinated participants in the previously uninfected cohort. There was evidence of considerable waning of protection more than 1 year after infection, with a reduction to 69%; protection during the first year after infection was 54% higher than that after more than 1 year.

In the previously infected cohort, with unvaccinated participants in the previously uninfected cohort as the reference group, a beneficial boosting of infection-acquired immunity was apparent, with combined protection of more than 90% after vaccination (after both the first and second doses). Waning of protection was not observed more than 1 year after infection or more than 6 months after vaccination.

BACKGROUND

The duration and effectiveness of immunity from infection with and vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are relevant to pandemic policy interventions, including the timing of vaccine boosters.

METHODS

Study investigated the duration and effectiveness of immunity in a prospective cohort of asymptomatic health care workers in the United Kingdom who underwent routine polymerase-chain-reaction (PCR) testing. Vaccine effectiveness (≤10 months after the first dose of vaccine) and infection-acquired immunity were assessed by comparing the time to PCR-confirmed infection in vaccinated persons with that in unvaccinated persons, stratified according to previous infection status. Study used a Cox regression model with adjustment for previous SARS-CoV-2 infection status, vaccine type and dosing interval, demographic characteristics, and workplace exposure to SARS-CoV-2.

RESULTS

Of 35,768 participants, 27% (9488) had a previous SARS-CoV-2 infection. Vaccine coverage was high: 97% of the participants had received two doses (78% had received BNT162b2 vaccine [Pfizer-BioNTech] with a long interval between doses, 9% BNT162b2 vaccine with a short interval between doses, and 8% ChAdOx1 nCoV-19 vaccine [AstraZeneca]). Between December 7, 2020, and September 21, 2021, a total of 2747 primary infections and 210 reinfections were observed. Among previously uninfected participants who received long-interval BNT162b2 vaccine, adjusted vaccine effectiveness decreased from 85% 14 to 73 days after the second dose to 51% at a median of 201 days after the second dose; this effectiveness did not differ significantly between the long-interval and short-interval BNT162b2 vaccine recipients. At 14 to 73 days after the second dose, adjusted vaccine effectiveness among ChAdOx1 nCoV-19 vaccine recipients was 58% considerably lower than that among BNT162b2 vaccine recipients. Infection-acquired immunity waned after 1 year in unvaccinated participants but remained consistently higher than 90% in those who were subsequently vaccinated, even in persons infected more than 18 months previously.

CONCLUSIONS

Two doses of BNT162b2 vaccine were associated with high short-term protection against SARS-CoV-2 infection; this protection waned considerably after 6 months. Infection-acquired immunity boosted with vaccination remained high more than 1 year after infection.