Individuals who recovered from COVID-19 and then received at least one shot of the Pfizer mRNA COVID-19 vaccine had a significantly lower risk of reinfection, according to a new study published Wednesday’s New England Journal of Medicine,.

The study was conducted by researchers at Israel’s Clalit Health Services, in collaboration with Ben-Gurion University and Sapir Academic College.

Specifically, the research showed there was an 82% reduction in the risk of reinfection with the virus among recovered and vaccinated 16- to 64-year-olds, and a 60% risk reduction among those aged 65 and older.  Contrary to recommendations in some countries, however, the study also suggested that one shot was sufficient in preventing re-infection with two shots performing no better.

“The results of our study unequivocally show that giving a coronavirus vaccine to people who have recovered is significantly associated with reducing the risk of re-infection,” said Dr. Doron Netzer of Clalit Health Services. However, she said the findings only covered peoples’ risks of re-infection during the Delta wave, and not necessarily Omicron. The latter variant has been less deadly but also more resistant to vaccine-induced protection.

The researchers noted that the results were consistent with data from studies that have shown strong immunologic responses to vaccination in previously infected persons.

Study applicable to Delta not Omicron

The study was conducted from March 1, 2021, the date on which Israel’s Ministry of Health approved the vaccination of people who recovered from COVID-19, through November 26, 2021 – at the height of the Delta wave. Although the study thus only covers the Delta wave, it also provided an opportunity to draw out some general principals on COVID vaccines, the researchers said.

“The surge of the Delta variant provided an epidemiologic opportunity to assess whether the vaccination of patients who had recovered from COVID-19 would provide extra protection against recurrent infection,” the researchers explained in their article.

The team analyzed the data of Clalit patients using advanced statistical methods to test the effectiveness of the vaccine in preventing re-infection with the virus among Covid-recovered people  who received the vaccine at least three months after being ill. their infection. They compared reinfection rates among patients who had subsequently received the vaccine and those who had not.

Clalit is Israel’s largest health fund.

In total, 149,032 people over the age of 16 who had recovered from coronavirus were included in the study – 83,356 who received at least one shot after infection. Reinfection occurred in 354 of the vaccinated patients (2.46 cases per 100,000 persons per day) and in 2,168 of the 65,676 unvaccinated patients (10.21 cases per 100,000 persons per day).

“Although vaccine effectiveness was lower among patients who were 65 years of age or older than among younger patients, the vaccine still offered substantial protection among older patients,” the researchers wrote. “However, among the unvaccinated patients, the reinfection rate among the older patients was much lower than that among the younger patients (3.02 cases per 100,000 persons per day vs.10.79 cases per 100,000 persons per day). This observation may be explained if we assume that older patients who had already been infected with SARS-CoV-2 would have observed enhanced social distancing and other required precautions, especially during the surge of the Delta variant, even if they had decided against vaccination.

“Therefore, the differences in reinfection rates between vaccinated and unvaccinated older patients were lower than those in the younger population,” they concluded.

One dose is enough

The research did not show a significant difference in vaccine effectiveness for one dose as compared with two doses.

That is a striking finding in light of the fact that many countries required two COVID jabs, even among recovered people, after a certain interval of time, to gain access to COVID vaccine passes that facilitated entry to leisure sites and travel. Though in Israel, there was a one shot recommendation at the time of the study, and to date, people who have recovered and have received only one vaccine dose before or after recovery are eligible for the pass.

“If you recovered from COVID-19, it is like you got a primary vaccination,” said Dr. Ronen Arbel, the senior researcher and the head of the Maximizing Health Outcomes Research Lab at Sapir Academic College. “It would be best to get vaccinated to boost the infection-induced immunity, but one shot is enough.”

Image Credits: Maccabi Health Services.