COVID vaccines offer long-lasting protection against reinfection, studies show

A pair of studies yesterday in the New England Journal of Medicine (NEJM) suggest good durable protection of COVID-19 vaccines against recurrent infections.

Fewer reinfections in older participants

Researchers from Clalit Health Services in Tel Aviv, Israel, looking back to analyse electronic health records of 83,356 recipients of at least one dose of the Pfizer/BioNTech COVID-19 vaccine after recovery from infection and 65,676 unvaccinated survivors.

The observational study began on March 1, 2021, after the Israeli Ministry of Health approved vaccination against COVID-19 for all patients who had recovered from COVID-19 3 months or more previously. All participants had recovered from primary SARS-CoV-2 infection from August 23, 2020 (190 days before the study period) to May 31, 2021 (90 days after the start of the study). The average age of the patients was 39.3 years (range, 16 to 110).

Reinfection with COVID-19 occurred in 354 of 83,356 vaccinated participants (0.4%; 2.5 cases per 100,000 person-days) and in 2,168 of 65,676 of their unvaccinated peers (3.3 %; 10.2 per 100,000).

In the 16-64 age group, 326 of 73,972 vaccinated participants (0.4%; 2.6 cases per 100,000 person-days) were re-infected, compared to 2,120 of 60,877 of their unvaccinated counterparts (3.5%; 10.8 per 100,000).

Among patients 65 years of age and older, reinfection occurred in 28 of 9384 vaccinated participants (0.3%; 1.5 cases per 100,000 person-days) and 48 of 4799 unvaccinated participants (1.0 %; 3.0 per 100,000). The study authors said the difference between the two age groups may be explained by the assumption that older COVID-19 survivors would have taken more precautions against reinfection than younger people.

According to a Cox proportional hazards regression model analysis, the adjusted hazard ratio (aHR) for reinfection in the vaccinated group, compared to the unvaccinated, among 16-64 year olds was 0.18. Among people 65 years and older, the aHR was 0.40. The estimated vaccine effectiveness in the youngest age group was 82%, compared to 60% in the older group.

A secondary analysis showed that the aHR for reinfection among the 67,560 participants who received one dose of COVID-19 vaccine, versus 15,251 who received two doses, was 0.98. “Given previous exposure to the virus, it appears that the primary vaccine dose in recovered patients provided a more robust and longer lasting immunogenic response than the first dose alone in patients with unprecedented Covid-19,” wrote Researchers.

The authors said the study, which covered the outbreak of the Delta variant in Israel, “supports a public health policy of vaccinating patients who have recovered from Covid-19, especially in places where the variant delta is still of concern”.

90% long-lasting protection when vaccine follows infection

A prospective study led by researchers from the UK Health Security Agency assessed the effectiveness and duration of vaccination against COVID-19 in a group of asymptomatic UK healthcare workers who were tested for infection every 2 weeks , as well as monthly antibody tests.

The team compared the time to SARS-CoV-2 infection in unvaccinated participants and those who received the Pfizer or AstraZeneca/Oxford COVID-19 vaccine up to 10 months previously, stratified by whether they had already been infected. The median age was 46 years and 84% were female.

Among 35,768 participants, 27% had recovered from COVID-19, as evidenced by the presence of anti-SARS-CoV-2 antibodies. Almost all participants (97%) had received two doses of vaccine; 78% of them had received the Pfizer vaccine with a long interval (6 weeks or more) between doses, while 9% had received the same vaccine with a short interval (less than 6 weeks) between doses, and 8 % had received the AstraZeneca vaccine.

From December 7, 2020 to September 21, 2021, there were 2,747 primary SARS-CoV-2 infections and 210 reinfections. Among COVID-19-naïve participants in the Pfizer long-interval group, adjusted vaccine efficacy fell from 85% at 14 to 73 days after the second dose to 51% at a median of 201 days. Efficacy was not significantly different between the long and short interval groups.

Among the AstraZeneca vaccinees, the adjusted vaccine efficacy was 58% after the second dose, much lower than that of the Pfizer vaccinees.

Unvaccinated participants saw their infection-acquired immunity decline after 1 year, although efficacy remained above 90% in those who were vaccinated after infection, even in those infected more than 18 months previously.

Overall, 357 participants (13%) with primary infection reported a hospital visit for COVID-19, compared to 18 (9%) of those who were reinfected.

“Two doses of BNT162b2 vaccine were associated with high short-term protection against SARS-CoV-2 infection; this protection declined significantly after 6 months,” amid the UK Delta surge, the authorities said. authors. “Vaccination-enhanced infection-acquired immunity remained elevated more than a year after infection.”

They conclude: “The strategic use of vaccine booster doses to avoid waning protection (particularly in doubly vaccinated, previously uninfected individuals) may reduce infection and transmission in the ongoing response to Covid-19. 19”.

Sequence of infection, vaccination may matter

In one audio interview in the same issue on the two studies, as well as another on the antiviral combination nirmatrelvir-ritonavir in COVID-19 patients, Eric Rubin, MD, PhD, NEJM editor and associate editor Lindsey Baden, MD, said the immune response may vary depending on the strain of SARS-CoV-2 that caused the natural infection and whether the natural infection or vaccination s is produced first.

“The sequence of these events — vaccination followed by natural infection or natural infection followed by vaccination — can have very different consequences and implications for immune augmentation and then subsequent immune enhancement,” Rubin said.

He said COVID-19 survivors may still benefit from subsequent vaccination, although the ideal time to vaccinate is still unknown: “There is a benefit, and although the absolute risk difference may be small, it It’s real. Also, there doesn’t appear to be a security issue with being boosted.”

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