Researchers suspect T-cells have such a large role in protecting the public from COVID -19 that herd immunity could be achieved at a 20 percent rate rather than the 60 to 80 percent assumed for other infectious diseases.
T-cells are a type of white blood cell that help individuals fight bacteria or viruses and may explain why some people who have been exposed to COVID-19 have never gotten sick. Researchers believe some individuals have acquired these cellular warriors by being exposed to coronaviruses in the past.
According to the U.S. Health and Human Services Public Data Hub, there have been more than 5.3 million confirmed cases of COVID-19 in the United States—1.6 percent of the population—and 168,696 deaths, as of August 17.
How T-Cells Work
T-cells work in a targeted way, says Yorgo Modis, Ph.D., a Wellcome Trust Senior Research Fellow at the Molecular Immunity Unit, Department of Medicine at the University of Cambridge.
“Once they have learned to recognize a certain pathogen, the T-cells become able to trigger an immune response against the pathogen, essentially by ringing the alarm bells of the immune system and recruiting all sorts of different types of immune cells that can kill the pathogen,” Modis told Health Care News.
For that reason, some of these T-cells are called killer cells, Modis says.
“It takes about two weeks for the T-cells to learn to recognize a new pathogen the first time we are exposed to it—sometimes longer, depending on the pathogen,” Modis said. “So without previous exposure, people are generally most vulnerable to new infections during the first two to three weeks. But once T-cells have learned to recognize a pathogen, they will remember the pathogen for a long time—often for a lifetime.”
That may explain why patients who die of COVID-19 typically die a few weeks after being infected. This recent exposure to a pathogen in the coronavirus family also sheds light on why some people have mild symptoms of COVID-19, while others end up on a ventilator.
“Each person does have a large and unique set of T-cells that have learned to recognize a different set of pathogens,” Modis said. “It purely depends on which pathogens the T-cells were exposed to during the person’s lifetime. Some people are thought to have some level of immunity from previous infections by other coronaviruses, or viruses from other families. There are various risk factors that are associated with a worse disease outcome—for example, but not limited to, diabetes. Some genetic associations have been reported in the scientific literature- people with certain genes, or multiple copies of a gene, [seeming to present] a higher risk. This is still an active area of research.”
The Herd Immunity Mystery
Pre-existing immunity may help the United States reach “herd immunity,” says Anne Marie Knott, a professor at Washington University.
“New COVID cases have peaked in most European nations and U.S. states, implying we’ve reached herd immunity,” Knott said. “We wouldn’t reach herd immunity until 58.3 percent of the population is immune. To date, only 1.6 percent of the U.S. population has contracted COVID-19, and we don’t have a vaccine, so we can’t have reached herd immunity through COVID itself.”
This could mean that many people already have some degree of immunity to coronaviruses, including COVID-19, Knott says.
“A recent paper in Cell documents that 40 to 60 percent of unexposed subjects exhibited immune responses to past antigens that appear to protect against COVID-19,” Knott said. “Another more recent paper, in Nature, detected T-cells that protect against COVID in 35 percent of healthy donors.”
Knott says the authors used a key phrase, stating immunity was “probably generated during past encounters with endemic coronaviruses.”
A Vaccine Booster
Until a vaccine is developed, T-cell research could help doctors treat COVID patients with T-cell therapy.
“Drugs that can control T-cell activity might help in treatment,” Modis said. “Sometimes, T-cells can cause the immune system to overreact and cause more damage than the virus itself. We don’t know what determines when this happens, but if we had ways to control or fine-tune T-cell activity, that should help us treat certain COVID-19 patients.”
The best bet is a vaccine because it would help protect those who are vulnerable to COVID-19 while posing little to no risk to people with some degree of immunity, Modis says.
“Herd immunity is when enough people within a population are immune that the virus can no longer spread exponentially,” Modis said. “Typically, this requires more than 60 percent of the population to be immune to the virus. There is no obvious reason why we can’t attain herd immunity against coronavirus, but it will require at least two-thirds of the population to either receive an effective vaccine or get infected by the virus and make a full recovery. So it is likely that we can get to herd immunity, but it will require people to get vaccinated.”
Fight May Never End
While T-cell immunity is being discussed in scientific journals, it is getting limited exposure in the general public.
In an August 15 article on the Marginal Revolution website entitled “The T-Cell Immune Response that Didn’t Bark,” Tyler Cowen, an economist at George Mason University, states one reason could be political incentives at work.
“If you do public health, your status incentives are to deliver warnings, not potential good news,” Cowen wrote. “Your status incentives are always to hedge your bets, and to be reluctant to introduce new hypothesis. Your status incentives are to steer talk away from the virus ‘simply continuing to rip,’ even if you are quite opposed to that outcome.
Your status incentives are to discourage individuals from thinking that they might have some pre-existing level of protection. That might lead them to behave more irresponsibly, and then you, in turn would look less responsible.”
Ashley Herzog (firstname.lastname@example.org) writes from Avon Lake, Ohio.