Unvaccinated individuals could expect to get reinfected with Covid-19 roughly every 16 months due to waning immunity, a study has found.
Yale School of Public Health analysed data for coronaviruses similar to Covid-19 including SARS-CoV-1 and Middle East respiratory syndrome (MERS-CoV), to estimate the likely reinfection rate for Covid.
Despite mounting reports of reinfection for Covid-19, the numbers are still not high enough for a proper study, so scientists analysed data from similar diseases to predict how long immunity will last.
The research published in the Lancet Microbe this month estimated immunity in those who got Covid-19 weakened fairly quickly.
Yale research associate Hayley Hassler, one of the study's co-authors, told Yale Daily News that the risk of reinfection was about 5 per cent at three months after the initial infection, and this rose to 50 per cent after 17 months.
People could get reinfected anywhere between three months to five years – with a median of 16 months.
The figures assume that Covid is circulating in the community and no restrictions such as masks or social distancing are in place.
The study also noted immunity could vary significantly among individuals, this could depend for example, on whether the person was asymptomatic, which could produce a weaker immune response than if they did get symptoms.
"Individuals may experience longer or shorter durations of immunity depending on immune status, cross-immunity, age and multiple other factors," Hassler said.
As the pandemic continues, scientists believe reinfection is likely to become increasingly common.
The study notes that maintaining social distancing measures, even for those who have already been infected, and speeding up the vaccination of the world would be critical to preventing deaths.
"We need to be very aware of the fact that this disease is likely to be circulating over the long term and that we don't have this long term immunity that many people seem to be hoping to rely on in order to protect them from disease," Yale professor of biostatistics and the study's lead author, Jeffrey Townsend, said.
The findings are also significant for reaching herd immunity. The study "argues strongly" against trying to achieve this without vaccination.
"Relying on herd immunity without widespread vaccination jeopardises millions of lives, entailing high rates of reinfection, morbidity, and death," the study stated.
In areas where vaccination is low, the study points to the need for continued social distancing, proper indoor ventilation, and mask wearing to avoid reinfection.
"The major implications are that if you haven't been vaccinated, you should get vaccinated, and if you've been infected, you should go ahead and get vaccinated anyway, because that will extend the duration of your protection," Townsend told the Guardian.
It comes as research published in the Lancet this month found two doses of the Pfizer vaccine were 90 per cent effective against hospitalisation for all variants, including Delta, for at least six months.
However, protection against infection from Delta fell from 93 per cent at one month, to 53 per cent after four months.
Effectiveness against hospitalisation remained high, at 93 per cent, over the eight months of the study.