There has been a lot of discussion on herd immunity to COVID-19 lately as new studies suggest that people’s immune system has the ability to recognize the virus, explaining why certain individuals have only mild symptoms or none at all.
Herd immunity happens when a large number of people are immune to a disease—either from prior infection or vaccination—which acts to stop or slow down the spread, thus protecting the entire community, including those who have not had the illness.
Estimates for percentages needed for herd immunity has mainly focused on the role of antibodies of those who have recovered from COVID-19, although scientists are still unsure how long the antibodies protect against reinfection.
Studies are now also beginning to consider the contribution of T-cells to herd immunity for COVID-19.
T-Cell Immunity for COVID-19
Director of the National Institutes of Health Dr. Francis Collins said in his blog post that studies on the immune response to SARS-CoV-2, the virus that causes the COVID-19 disease, have concentrated mostly on the body’s production of antibodies to determine if we will have immunity against the disease.
“But, in fact, immune cells known as memory T-cells also play an important role in the ability of our immune system to protect us against many viral infections, including—it now appears—COVID-19,” Collins said.
T-cells (along with B-cells) are a type of white blood cell that plays an essential part in the adaptive immune system by assisting the body in destroying and remembering antigens—foreign substances that invade the body. Although T-cells are produced in the bone marrow, they travel to the thymus, a small organ between the lungs directly behind the sternum in the chest, where they will mature into different types of T-cells with specific roles.
As people get older, the thymus shrinks, affecting the production and functionality of T-cells, and so adults 60 and older become more susceptible to infections. The elderly population has been the most affected in the pandemic, where at least 45 percent “of all COVID deaths are coming from 0.62 percent of the population—those in nursing homes and assisted living facilities,” Dr. Tom Reed, a double board-certified medical and surgical specialist, told a Facebook discussion hosted by Texas state Sen. Bob Hall.
A study published in Nature that Collins cites in his blog suggests that the immune system’s T-cells may offer protection against COVID-19 by “remembering past encounters with other human coronaviruses,” explaining why certain people can fight off the illness or experience only mild symptoms.
There are six coronaviruses besides SARS-CoV-2 that infect humans—four of them cause the common cold, while two caused the 2003 Severe Acute Respiratory Syndrome (SARS) outbreak and the 2012 Middle East Respiratory Syndrome (MERS) outbreak. These coronaviruses produced both memory T-cells and antibodies in infected individuals, but it was the memory T-cells that continued to confer immunity for over a decade, according to the study.
Multiple studies have been published since the Nature study confirming similar results.
The latest one to be published in Cell (pdf) shows a more promising role for T-cells in developing herd immunity against COVID-19.
Researchers found that “donors exhibited robust memory T-cell responses months after infection, even in the absence of detectable circulating antibodies specific for SARS-CoV-2, indicating a previously unanticipated degree of population-level immunity against COVID-19.”
Dr. Scott Atlas, President Donald Trump’s new advisor on the pandemic, said last month that emerging data showed that people who didn’t get infected have been found to have COVID-19 immunity. “This is probably due to this T-cell immunity, which is present, and is now shown to last for years,” he said.
Atlas, speaking during an appearance on Fox News, pointed to research from Singapore and from Sweden’s Karolinska Institute. Researchers from the institute stated that many people with mild or asymptomatic cases of COVID-19 demonstrate T-cell-mediated immunity to SARS-CoV-2.
Achieving Herd Immunity
It is generally thought that an estimated 60 to 70 percent of the population needs to become infected by SARS-CoV-2 before herd immunity is achieved. “However, several recent studies have suggested that depending on specific population factors (population density, propensity for travel, susceptibility, etc.), this number may be considerably lower in some sub-populations in particular places around the country,” Dr. Michael Devine, dual-board certified internist and geriatrician and co-founder of Devine Concierge Medicine, told The Epoch Times in an email.
But when asked if some states are already seeing herd immunity, Devine says he doesn’t believe that is the case just yet.
“In every measure we do have at present, the perceived proportion of the population who is believed to have acquired (by having been infected directly) or innate (naturally occurring or genetically predisposed) immunity falls vastly short of ..."