The study, published Jan. 6 in Science, contrasts with earlier findings suggesting immunity to covid-19 could be short lived, putting millions of people who have already recovered at risk of reinfection. This difficult situation would not have come as a total surprise, since the infection with other coronaviruses generates antibodies that fade quite quickly. But the new study suggests that reinfection should only be a problem for a very small percentage of people who have developed immunity – whether through an initial infection or through vaccination.
In fact, the new study shows that a small number of recovered people do not have lasting immunity. But vaccination should compensate for this problem by ensuring herd immunity to the entire population.
The new article looked at blood samples from 185 men and women who had recovered from covid-19 – most from mild infection, though 7% were hospitalized. Each person provided at least one blood sample between six days and eight months after their first symptoms, and 43 of the samples were taken after six months. The team that carried out the investigation measured the levels of several immunological agents that work together to prevent reinfection: antibodies (which mark a pathogen for destruction by the immune system or neutralize its activity), cells B (which produce antibodies) and T cells (which kill infected cells).
The researchers found that the antibodies in the body decreased moderately after eight months, although the levels varied widely between individuals. But the number of T cells decreased only modestly, and the number of B cells remained stable and sometimes increased inexplicably. This means that despite the decrease in free-flowing antibodies, the components that can restart antibody production and coordinate an attack against the coronavirus remain at fairly high levels. Crotty adds that the same mechanisms that lead to immune memory after infection also form the basis of immunity after vaccination, so the same trends should apply to those vaccinated as well.
And while immunity to other coronaviruses has been less than stellar, it’s worth looking at what happens in people who have recovered from SARS, a close cousin of the virus that causes covid-19. A study published in August showed that SARS-specific T cells can remain in the blood for at least 17 years old, reinforcing hope that immunity to covid-19 could last for decades.
The new study is not perfect. It would have been better to take several blood samples from each participant. “Immunity varies from person to person, and rare individuals with poor immune memory may still be susceptible to reinfection,” Crotty warns. And we can’t draw definitive conclusions about covid-19 immunity until years have passed – it’s just too early. Nevertheless, this last result is a good indication that if the deployment of the vaccination is going well (a large if), we may soon be able to put the pandemic behind us.