Don't panic about waning coronavirus immunity: Your T cells go to bat when antibodies disappear

Ameer Al Mohammedaw/GettyA recovered coronavirus patient donates blood samples for plasma extraction to help critically ill patients at the National Blood Transfusion Centre on June 22, 2020.

A handful of recent studies have painted a worrisome picture about immunity to the coronavirus: Early research suggests these protective proteins can fade within weeks or months.

But antibodies don’t tell the whole story.

Your protection against reinfection is wrapped up in the immune system’s multi-layered response to an invading virus. White blood cells have impressive powers of recollection that can help your body mount another attack against the coronavirus should it ever return: Memory T cells can identify and destroy infected cells, and inform B cells about how to craft new virus-targeting antibodies.

“These so-called memory cells are the main agents of long-term immunity,” two immunobiologists, Akiko Iwasaki and Ruslan Medzhitov, wrote in The New York Times on Friday. When it comes to antibodies, however, the level “in the blood peaks during an infection and drops after the infection has cleared, often within a few months,” they added.

A recent study, published July 15, examined 36 recovered coronavirus patients and found that all of them produced memory T cells that recognise and are specifically engineered to fight the virus. Those T cells will go to bat whether or not antibodies are still at detectable levels.

“The advantage of having really good T cells is even if the antibody levels have gone down, every time you get exposed again, the T cells will clone up and provide help really quickly, and those antibodies will be expanded again,” Dr. Richard Locksley, an infectious-disease expert, told Elemental.

‘There’s no evidence that immunity is fleeting’

Research is coalescing around the idea that people who catch COVID-19 develop robust T cell responses.

A study published Wednesday in the journal Nature found that among 18 German coronavirus patients, more than 80% developed virus-specific T cells.

Similarly, research published in May found that all members of a group of 20 coronavirus patients who’d gotten mild or moderate infections had helper T cells capable of recognising the virus and responding accordingly. About 70% made killer T cells as well. A follow-up study also looked at 10 patients with severe COVID-19 cases that required hospitalisation and found that all 10 produced helper T cells and 80% produced killer T cells.

“The data are suggestive that the average person makes a good immune response and may have immunity for some time,” Shane Crotty, a coauthor of that May study, previously told Business Insider.

Thomas Peter/ReutersA tube with a solution containing COVID-19 antibodies.

Marm Kilpatrick, a disease ecologist at University of California Santa Cruz, previously told Business Insider that “there’s no evidence that immunity is fleeting and no rigorous estimates yet of how long it will last.”

He added that the limited data available (which mostly only goes up to 60 days after infection) “shows waning of some immune markers but not immunity more broadly.”

People who’ve had other coronaviruses may have a ‘head start’ against COVID-19

The July 15 study yielded a second, more surprising finding as well: Among 37 healthy people who had never gotten COVID-19, more than half had memory T cells that could recognise the new coronavirus.

“This might potentially explain why some people seem to fend off the virus and may be less susceptible to becoming severely ill with COVID-19,” Dr. Francis Collins, director of the National Institutes of Health, wrote in a blog post about the findings.

The researchers also examined 23 people who had survived SARS, which is a coronavirus, too. The results showed that those survivors still had SARS-specific memory T cells 17 years after getting sick, and also that those same T cells could recognise the new coronavirus.

People wear masks as protection against the SARS virus as they wait to buy tickets at the Beijing Railway Station Wednesday, April 23, 2003.Greg Baker/ APPeople wear masks as protection against the SARS virus as they wait to buy tickets at the Beijing Railway Station, April 23, 2003.

The Nature study in German patients found similar results: In a cohort of 68 healthy people who’d never had COVID-19, more than one-third had T cells that reacted to the virus.

Crotty’s research told the same story in May. Twenty people who had never been exposed to the new coronavirus were found to have helper T cells capable of recognising and responding to it.

The likeliest explanation for these findings is a phenomenon called cross-reactivity: when T cells developed in response to another virus react to a similar, but previously unknown, pathogen. In this case, these cross-reactive T cells likely come from our previous exposure to common colds.

The findings suggest that our immune system could lean on its past experiences fighting other coronaviruses to better battle the new one.

T cellNIAIDA human T lymphocyte (also called a T cell) from the immune system of a healthy donor.

“You’re starting with a little bit of an advantage – a head start in the arms race between the virus that wants to reproduce and the immune system wanting to eliminate it,” Alessandro Sette, a coauthor of Crotty’s, previously told Business Insider.

A large chunk of people may have this head start.

White blood cells from 20% to 50% of unexposed people significantly react to the new coronavirus, Sette and Crotty wrote in a follow-up analysis published last month, adding, “pre-existing immune reactivity exists to some degree in the general population.”

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