Not one of more than 1,300 unvaccinated employees who had been previously infected tested positive during the five months of the study. Researchers concluded the cohort “are unlikely to benefit from COVID-19 vaccination.”

In the largest real-world observational study comparing natural immunity gained through previous SARS-CoV-2 infection to vaccine-induced immunity afforded by the Pfizer vaccine, researchers in Israel found people who recovered from COVID were much less likely than never-infected, vaccinated people to get Delta, develop symptoms or be hospitalized.

“Our results question the need to vaccinate previously infected individuals,” they concluded.

Experts speak out on natural immunity

In a recent letter to the editor of The BMJDr. Manish Joshi, a pulmonologist at UAMS Health; Dr. Thaddeus Bartter, a pulmonologist at UAMS Health; and Anita Joshi, BDS, MPH, said data demonstrate both adequate and long-lasting protection in those who have recovered from COVID, while the duration of vaccine-induced immunity is not fully known.

The authors of the letter said the “SIREN” study in the Lancet addressed the relationships between seropositivity in people with previous COVID infection and subsequent risk of severe acute respiratory syndrome due to SARS-CoV-2 infection over the subsequent seven to 12 months.

The study found prior infection decreased risk of symptomatic reinfection by 93%.

A large cohort study published in JAMA Internal Medicine which looked at 3.2 million U.S. patients, showed the risk of infection was significantly lower (0.3%) in seropositive patients compared to those who were seronegative (3%).

A recent study published in May in the journal Nature demonstrated the presence of long-lived memory immune cells in those who have recovered from COVID-19 suggesting durable and long-lasting immunity.

“This implies a prolonged (perhaps years) capacity to respond to new infection with new antibodies,” the authors wrote.