Washington, Oct 20 : If you think once infected with Covid-19 gives you immunity, then you may be wrong.
According to new models without vaccination and masks, people once infected with Covid can be reinfected within four months, Nature reported.
Within four months after initial infection, the average reinfection risk rises to about 5 per cent.
The risk can increase upto 50 per cent by 17 months, the report said. At the same time, natural immunity was found to last for less than half as long as it does for the common-cold coronaviruses.
The predictions are based on the genetic relationships between SARS-CoV-2 and other coronaviruses.
"Immunity is relatively short-lived.
You should still get vaccinated even if you got infected," Jeffrey Townsend, a bioinformatician at the Yale School of Public Health in New Haven, Connecticut was quoted as saying.
While more data over the coming months, and years, will be necessary to know precisely how long natural immunity lasts, it's not necessary "to wait for that", Townsend said.
To estimate the durability of SARS-CoV-2 immunity, the team wanted to understand how antibody levels from a previous infection affect the risk of reinfection.
They combined genetic data from SARS-CoV-2, three endemic coronaviruses that cause the common cold, and the closely related coronaviruses SARS-CoV and MERS-CoV to build a viral family tree.
Using this, they modelled how viral traits have evolved over time.
Together, these traits provided an estimate of the decline in antibody levels after SARS-CoV-2 infection, and of other factors needed to understand reinfection risk.
The findings suggest that Covid-19 is likely to transition from a pandemic disease to one that is endemic, Townsend said.
Still, many unknowns remain, including the probable severity of disease when someone is reinfected.
Individuals can also vary significantly in both their susceptibility to reinfection and, if reinfected, their disease course -- including whether they are likely to get long Covid, the report said.