Natural immunity induced by infection with SARS-CoV-2 provides a strong shield against reinfection by a pre-Omicron variant for 16 months or longer, according to a study1. This protection against catching the virus dwindles over time, but immunity triggered by previous infection also thwarts the development of severe COVID-19 symptoms — and this safeguard shows no signs of waning.
The study1, which analyses cases in the entire population of Qatar, suggests that although the world will continue to be hit by waves of SARS-CoV-2 infection, future surges will not leave hospitals overcrowded with people with COVID-19. The research was posted on the medRxiv preprint server on 7 July. It has not yet been peer reviewed.
The study is “solid”, says Shane Crotty, an immunologist at the La Jolla Institute for Immunology in California. “The data make sense and are in line with multiple other studies and previous work by this group.”
Better late than never
But scientists also warn that the study’s results do not mean that infected people can skip vaccination. A separate study2 by many of the same authors found that “people who had both natural immunity and vaccine immunity were substantially more protected against the virus than people who had only natural immunity alone or vaccine immunity”, says Laith Abu-Raddad, an infectious-disease epidemiologist at Weill Cornell Medicine–Qatar in Doha and a co-author of both studies. “It was very clear-cut.”
Studies3,4 on the effectiveness of COVID-19 vaccines suggest that protection against the virus SARS-CoV-2 decreases over time, waning considerably after six months. To learn about the course of naturally acquired immunity, the authors examined COVID-19 data gathered in Qatar between 28 February 2020 and 5 June 2022. “Our study is the first to have such a long time of follow-up,” says co-author Hiam Chemaitelly, an epidemiologist also at Weill Cornell Medicine–Qatar.
The researchers compared COVID-19 cases in unvaccinated individuals who’d had one previous SARS-CoV-2 infection with cases in unvaccinated people who’d never previously caught the virus. They found that infection with a pre-Omicron variant prevented reinfection by another pre-Omicron variant with an average effectiveness of 85.5% for the period covering the 4th through the 16th month following the initial infection.
Effectiveness peaked at 90.5% in the 7th month after the first infection and fell to about 70% at 16 months (see ‘Immunity fades away’). By extrapolating this trend, the authors predict that effectiveness against reinfection will fall to less than 10% 32 months after the first infection.
Pre-Omicron infection was only 38% effective at preventing infection by an Omicron variant in the first 6 months after Omicron emerged. Modelling suggests that the number will drop to 10% at 15 months.
All the same, infection with any SARS-CoV-2 variant is highly effective at combating severe, critical or fatal COVID-19 after reinfection: effectiveness was around 100% up to the 14th month after primary infection and showed no signs of declining.
Old and young alike
The authors note that most of Qatar’s population is young, so the findings might not apply to populations with a higher average age. But when the team restricted its analysis to people more than 50 years old, the levels of protection were similar.
Other potential caveats exist. The authors’ projections assume that the immune response changes at a specific rate, when in fact that rate depends on the length of time since a person caught the virus, says Crotty. Therefore, immune responses measured at one point in time might not allow accurate predictions of the future.
Regardless of the extrapolations, the data indicate that naturally acquired immunity is hardy — something that is not always championed.
“In the US, we were underselling the immune protection provided by previous infection,” says Jeffrey Morris, a biomedical data scientist at the University of Pennsylvania in Philadelphia. He adds that the Qatar team’s study affirms the substantial evidence for natural immunity’s capabilities.