Why ‘T cell immunity’ won’t end the coronavirus pandemic

A host of new studies on immune cells and Covid-19 are behind the latest misinterpretation of research on Covid-19 that is currently infecting social media, especially among President Trump’s political supporters.

An informal Twitter analysis shows some early research by scientists investigating the coronavirus and the immune system has been widely misinterpreted. These social media posts claim that thanks to “T cell immunity,” it will only take 10-20% of the population to become infected with SARS-CoV-2 to achieve herd immunity – the point where the disease will slowly stop its spread. According to some dangerously optimistic tweets, this means the pandemic could be over by October.

That certainly sounds good, because who doesn’t want the Covid-19 pandemic to be over? There’s only one problem: According to scientists, that’s not how it works at all. Far from preventing infection, T cell immunity can at best lead to a milder case of Covid-19 in individuals.

And when it comes to this illness, “less severe” can simply mean “not immediately hospitalized”. A recent study on The Journal of the American Medical Association found that some people who tested positive for Covid-19, but had mild or even no symptoms, had myocarditis, an inflammation of the heart that can lead to severe damage or even heart attacks. Which shows that many dangers of Covid-19 are still unknown.

Still, that doesn’t mean things are hopeless. To help clear things up, here is a look at the scientific answers to your questions about how T cells work, which recent research really tells us and what immunity against Covid-19 might look like.

What are T cells?

Think of your body as a medieval castle under siege. This castle protects itself by means of both defense and attack: the innate immune system and the adaptive immune system. Just like a castle has built-in defenses, such as a moat, walls, and a drawbridge, your innate immune system is your body’s bulwark against bacteria. Skin, mucous membranes, stomach acid, and other things are all designed to ward off attacks from the outside. If attackers breach your castle, your adaptive immune system acts like archers, spies, and knights against invaders. They may take a little longer to put on their armor, but once they fight they can use deadly precision. These are the B cells in your body which make antibodies and the T cells.

T cells are hyper-specialized white blood cells, with each type of T cell playing a specific role during infection. Some T cells directly kill invading cells, while other T cells help activate B cells and stimulate them to make antibodies. Still others become “memory cells,” which patrol the body for years after the initial infection to prevent reinfection of already defeated viruses or bacteria. If these memory cells encounter an enemy from the past, an immune response is activated. Some types of B cells can also be memory cells and quickly pump out antibodies if a recognized pathogen returns.

What is the connection between T cells and Covid-19 immunity?

SARS-CoV-2 is one of seven coronaviruses known to infect humans, four of which are widespread almost everywhere and cause colds and other respiratory infections. “Most people have been exposed [to a coronavirus] from early childhood, ”says John Wherry, an immunologist at the University of Pennsylvania.

These childhood infections lead to the creation of memory T cells, and several recent articles have shown that memory T cells from other coronaviruses recognize SARS-CoV-2, the virus that causes Covid-19. Not with much specificity – to use an analogy, T cells can recognize that this is an enemy soldier in uniform, but cannot tell what the soldier’s rank is or what his specialty is. Yet at least that is enough to sound the alarm: at least 20 to 50% of people who have not been exposed to Covid-19 may already have T cells that will activate and defend themselves against the disease.

Does this mean that 20 to 50% of people are already immune to Covid-19?

Some politicians, including new Covid-19 task force member Scott Atlas, have reportedly suggested that due to cross-immunity, the pandemic could be over as early as October. Scientists disagree.

“No, not correct,” Wherry said.

This is because a T cell recognizing the coronavirus doesn’t mean you still won’t be sick. T cells are only activated when the virus is already replicating inside a cell, so by the time the cross-reacting memory T cells are involved, you have already been infected. All the alarm sounds is your body’s immune system is working more efficiently, which means you could end up with less severe illness than you would have otherwise had. Emphasize “may”, because there is still a lot we don’t know.

“Even if our most optimistic speculations about the memory of cross-reactive T cells turned out to be correct,” says Shane Crotty, an immunologist at the La Jolla Institute of Immunology, “the most likely effect would not be a prevention of SARS-CoV- 2 infections. Instead, the memory of the cross-reacting T cells would reduce the severity of the disease, so that fewer people would fall seriously ill or die from COVID-19.

How does all of this relate to herd immunity?

Collective immunity means that once a certain percentage of people in a population are immune to Covid-19, the disease stops spreading in that population. Think of them like traffic lights: Even if there is a green light in front of you, you won’t be able to cross it if you keep running red lights along the way. Researchers believe that to stop the spread of Covid-19, around 70% of the population must be immune to the virus.

Recently, an idea caught on on social media suggesting that hard-hit regions, like New York or Sweden, have already reached the herd immunity threshold. But that’s not how it works, said Crotty. “Corners of the Twitterverse began to add up 20% of the population (for example, in New York City) who had recovered from Covid-19, plus the 50% of individuals who have cross-reactive T cells, and a falsely concluded that we have now achieved the herd immunity because 20% + 50% = 70%, ”he says, referring to some viral social media posts making the claim. “It’s just incorrect immunology.”

There are two ways to obtain collective immunity against Covid-19: either 70% of the population becomes ill, or 70% of the population receives a vaccine that protects against Covid-19. Currently, more than 160 groups around the world are working to develop a vaccine against Covid-19. No vaccine has yet been approved in the United States, but several companies, including Moderna and Pfizer, are testing their vaccines in Phase 3 clinical trials. Most experts predict that the first effective vaccine against Covid- 19 could be approved is 2021.

That said, a vaccine or even natural herd immunity is not a quick fix. Just like you need to get a flu shot every year, Covid-19 may also require regular injections to stay protected. And just as public health experts are constantly on the lookout for new, more dangerous forms of influenza, so could coronaviruses.

So what are we doing until 2021?

“If you want something that will aid herd immunity, our public health practices will,” says Wherry. This includes continued social distancing, wearing masks, and tracking areas with high disease transmission. Continue to wash your hands and while you are at it: get your flu shot. Experts warn that, as we move into flu season, the double punch of the flu and the coronavirus has the potential to increase the severity of both.

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