Among the many unknowns of the COVID-19 pandemic are the duration of immunity and the odds of reinfection. There’s still very limited information on the virus’ long-term immune responses and reinfection but now, a new study, suggests that people who have been infected with the virus can expect to catch it again in one or two years. 

Collecting data to establish the rate of reinfection of an infectious disease normally takes several years — and we haven’t really had the time with Covid-19, which has been circulating actively for just a year and a half. With this in mind, researchers at Yale University decided to look at immunological data from other coronaviruses, such as SARS-CoV-1. 

By understanding how other coronaviruses evolve and how they are connected to each other, the team could model the likelihood of reinfection for SARS-CoV-2. They identified traits across the other viruses and with this estimated the decline in antibody levels after infection, as well other factors needed to understand reinfection.

“The overall goal of the study was to provide an answer to a question that at this point in the pandemic would be impossible to answer empirically, which is how long after you’ve been infected by SARS-CoV-2 can you expect to possess immunity against the virus before you become vulnerable to reinfection?” Hayley Hassler, one of the study’s co-authors, said in a statement.

A very likely reinfection

The model projected the risk of reinfection only under endemic conditions, in which everybody has been infected or vaccinated. Overall, the study suggests that immunity from SARS-CoV-2 doesn’t really last a long time. The risk of reinfection is about 5% at four months after the initial infection (if exposed to the disease). After 17 months, the number goes up to 50%.

Natural immunity against coronaviruses in humans is usually short-lived. Even so, this is a much shorter immune period (about two times shorter) than other human coronaviruses. The fact that Covid-19 has more dangerous outcomes during and after infection is another reason for concern.

Some news coverage has compared SARS-CoV-2 to the flu and measles, suggesting that these viruses could provide similar types of immunity, but this is misleading, according to the researchers. The viruses aren’t that much related to each other and can’t be expected to have similar properties. A life-long immunity of Covid-19 should be ruled out, they added. We may be stuck with yearly vaccines for COVID-19, much like we have for the flu.

“As the COVID-19 pandemic continues, reinfection is likely to become increasingly common. Maintaining public health measures that curb transmission—including among individuals who were previously infected with SARS-CoV-2—coupled with persistent efforts to accelerate vaccination worldwide is critical to the prevention of COVID-19 morbidity and mortality,” the researchers wrote.

The study was published in the journal The Lancet Microbe.