Immunity against SARS-CoV-2 may last at least 8 months

A new study suggests that most people who recover from a SARS-CoV-2 infection retain the immune memory they would need to prevent a severe reinfection. The protection provided by vaccination may be similarly long-lasting.

• New research has found that people who have recovered from COVID-19 may have immunity to SARS-CoV-2 for 8 months or more.

• This is due to long-lasting immune memory that allows the immune system to remain active against the virus.

• However, due to differences in individual immune memories, not all people who have recovered from COVID-19 will have long-lasting immunity against the virus.

Researchers at the La Jolla Institute in La Jolla, CA, monitored all four branches of the “adaptive” immune system — which protects against specific infections — in people who recovered from COVID-19, the illness that the SARS-CoV-2 virus causes.

Between 5 and 8 months after the onset of their symptoms, the adaptive immune system of around 95% of these individuals retained a memory for the infection in at least three of its branches.

“Our data suggest that the immune response is there — and it stays,” says Prof. Alessandro Sette, who co-led the study with Prof. Shane Crotty.

The scientists measured levels of antibodies, memory B cells, and two types of T cells, all targeted at the virus.

“As far as we know, this is the largest study ever for any acute infection that has measured all four of those components of immune memory,” says Prof. Crotty.

The research appears in the journal Science.

Conflicting results

Medical News Today has reported several smaller investigations with apparently contradictory findings regarding post-infection immunity.

For example, researchers at the University of Arizona College of Medicine in Tucson found that people who had a COVID-19 infection were still producing antibodies against the virus 5–7 months later.

By contrast, a study by Imperial College London in the United Kingdom suggested that antibody responses to the virus within a population decline following the first wave of infection.

As a result, there have been concerns that many people’s immune response will be too weak to protect them from reinfection. This would also have implications for the longevity of protection afforded by vaccines.

“Of course, the immune response decreases over time to a certain extent, but that’s normal,” says Prof. Sette. “That’s what immune responses do. They have a first phase of ramping up, and after that fantastic expansion, eventually, the immune response contracts somewhat and gets to a steady state.”

Prof. Sette and his colleagues analyzed 254 blood samples from 188 individuals who recovered from COVID-19. Of these, 43 samples were taken at least 6 months after the infection.

They found that levels of antibodies targeted at the spike protein, which the virus uses to gain entry to its host cells, remained relatively stable for at least 6 months.

More surprisingly, perhaps, the memory B cells that produce these antibodies were more abundant 6 months after the onset of symptoms than they had been at 1 month.

These cells would likely ramp up their production of antibodies after a second encounter with the virus.

The researchers also measured levels of T helper cells, which recruit other parts of the immune system to fight an infection, and killer T cells, which destroy infected cells.

They found that 3–5 months after the onset of symptoms, the number of these two types of T cells that targeted the virus had roughly halved.

Team effort

The scientists believe it is a good sign that some antibodies, memory B cells, and T cells remain in a person’s blood more than 8 months after infection because the different branches of the adaptive immune system work together.

“This implies that there’s a good chance people would have protective immunity, at least against serious disease, for that period of time, and probably well beyond that,” says Prof. Crotty.

All the same, the study did find large variations in the magnitude of immune memory between individuals. This suggests that people with relatively weak adaptive immunity to the virus may remain susceptible to reinfection.

The authors caution:

“We observed heterogeneity in the magnitude of adaptive immune responses to SARS-CoV-2 persisting into the immune memory phase. It is, therefore, possible that a fraction of the SARS-CoV-2-infected population with low immune memory would become susceptible to reinfection relatively soon.”

They note that there have been some confirmed cases of reinfection. For example, there was a case reported in Hong Kong and another in the United States.

The authors say future studies should include more longitudinal data for each subject to gain a more precise understanding of the durability of antibody defenses against the virus.

In addition, they concede that local immune memory in the upper respiratory tract may play an important role in determining whether a person can be reinfected. The current study only measured immune cells and antibodies circulating in the bloodstream.

Finally, it is important to note that the researchers did not design the study to address the possibility that mutations could allow the virus to “escape” the adaptive immune response after an initial infection or vaccination.

Written by James Kingsland on January 11, 2021 — Fact checked by Zia Sherrell, MPH

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