The presence of pre-existing T cell memory against four common cold coronaviruses which cross-reacts with novel coronavirus may not prevent SARS-CoV-2 infection, but at most be able to reduce  the severity of COVID-19 disease and probably prevent people from dying. The T-cell memory does not allow populations to reach herd immunity when just 10-20% are infected.

Researchers at La Jolla Institute for Immunology, La Jolla, California had in first found that SARS-CoV-2-reactive T cell memory in 40–60% of people who have not been exposed to novel coronavirus. This “suggested cross-reactive T cell recognition between circulating common cold coronaviruses and SARS-CoV-2,” they wrote in a paper published in Cell. Four other preprints too suggested the presence of T cell memory from common cold coronavirus that were cross-reactive with SARS-CoV-2 in people who have not been exposed to novel coronavirus.

In another paper published in Nature, Dr Shane Crotty from La Jolla Institute for Immunology showed that the T cell memory were from previous common cold coronavirus infections. And in a third paper published in Science, Dr Shane Crotty and others from La Jolla Institute for Immunology “demonstrated a range of pre-existing memory T cells that are cross-reactive with comparable affinity to SARS-CoV-2 and the common cold coronaviruses”.

None of the three papers or the preprints had ever suggested even remotely that T cell memory from common cold coronaviruses that were cross-reactive with SARS-CoV-2 would help achieve herd immunity even at low percentage of infection in the community.

Misconception about herd immunity

But Dr James Mathew Todaro, an ophthalmologist at Columbia University, had on August 10 in a tweet extrapolated what was said in the papers and suggested that cross-reactive  T cell memory were allowing just that — allowing her immunity even at low percentage infections in the community. In the tweet, Dr Todaro says: “There is growing evidence that T-cell immunity allows populations to reach herd immunity once only 10-20% are infected with SARS-CoV-2. This would explain why a highly transmissible virus in densely populated areas peaked at 10-20% infected regardless of lockdowns or masks.”

Based on a wrong assumption, Dr Todaro goes on to accuse epidemiologists for suggesting high percentage infection required to reach herd immunity. He said: “The pervasive misconception is that we have zero immunity against COVID-19. Based on this flawed understanding, epidemiologists projected that herd immunity is not reached until 60-70% are infected. This is almost certainly wrong”.

Clearing the air

To clear the air and misconception regarding  T cell memory of common cold coronavirus that are cross-reactive with SARS-CoV-2 coronavirus and herd immunity, Dr Crotty, who is co-author in all three papers, says in a tweet: “There are various tweets misinterpreting COVID-19 “pre-existing immunity” and making dangerous claims about herd immunity,”

He says cross-reactive immune memory, which avoids the word “immunity”, may sometimes get misinterpreted as “protective immunity”. “The observations about pre-existing T cell immunity are important because these memory cells may impact people’s responses to SARS-CoV-2 infection, or COVID-19 vaccines,” Dr Crotty says.

Herd immunity is said to be achieved when a sizeable percentage of people in a community is immunised to the virus thus protecting the remining from getting infected. Dr Crotty then says: “Even if our most optimistic speculations about cross-reactive T cell memory were found to be correct, it would mean that just as many people would get infected with SARS-CoV-2, but fewer would become severely ill and die from COVID-19.”

And Dr Crotty adds: “T cells generally don’t completely prevent infections, they limit disease (make it shorter and/or less serious).”

Put simply, the presence of cross-reactive T cell memory may not prevent infection, but at most be able to reduce the severity of the disease and probably prevent people from dying.

Dr Crotty advocates wearing a mask to prevent getting infected and says masks are “much more effective than hoping you and the people around you have pre-existing T cell memory”.