President Donald Trump wears a face mask as he tours a medical lab on July 27, 2020.
  • US president Donald Trump claimed that is "immune" to the coronavirus in a Fox News interview on Sunday.
  • Though experts think that most people develop an immune response after a COVID-19 infection, it's unclear how strong this response is or how long the protection lasts.
  • There are no indicators that could reliably determine whether Trump is immune to reinfection.
  • Visit Business Insider's homepage for more stories.

US president Donald Trump claims he is immune to the coronavirus, but there's no way he can be sure of that.

"It looks like I'm immune for, I don't know, maybe a long time, maybe a short time," Trump said in a live Fox News interview with Maria Bartiromo on Sunday. "It could be a lifetime. Nobody really knows."

Trump also said he had "a protective glow" — a concept which does not appear in medical literature or scientific research about the virus.

A Saturday night note from the White House physician Sean Conley did not mention immunity. Conley wrote that Trump was no longer "a transmission risk to others," adding that he would continue to monitor Trump as the president "returns to an active schedule."

"Now you have a president who doesn't have to hide in a basement like his opponent. You have a president who is immune," Trump said. "Which is a very important thing, frankly."

But it's impossible to know whether an individual is truly immune to the virus. Could Trump get reinfected? If his body has launched a robust immune response, how long will it last?

The president is right about one thing: Nobody really knows.

There is no reliable indicator of COVID-19 immunity

Experts generally agree that most patients walk away from a coronavirus infection with a degree of immunity, but there is no way to know whether an individual has developed a strong enough immune response to ward off reinfection. The US Centers for Disease Control and Prevention reports that researchers have not yet established "correlates," or indicators, for immunity.

CDC guidelines do allow for an infected person to come out of isolation 10 days after they first show symptoms (as long as they have no fever without taking fever-reducing medications), but that's because 10-day patients are unlikely to shed enough virus to infect others. The agency has clarified that this does not imply that people are immune after 10 days of illness.

It's still unclear how strong the COVID-19 immune response is, how long it lasts, or what makes it vary between individuals. Some research indicates that immunity only lasts a few months, and there have been suspected cases of people getting reinfected several weeks after recovering from the virus. 

"We now know that reinfection can occur," John Wherry, an immunologist at the University of Pennsylvania in Philadelphia, told Nature in August. "It remains unclear how frequent of an event reinfection is, or what features of the immune response are associated with reinfection."

The most immediate way to measure an immune response is to test for antibodies — proteins that fight off viruses and can help prevent reinfection. On Wednesday, Conley said that Trump had tested positive for antibodies, though those could have come from the Regeneron experimental antibody cocktail he received on October 2.

Some research has suggested that antibodies can fade just a few weeks after infection. That's when white blood cells — the arbiters of long-term immunity — step in. These include B cells, which produce antibodies, and memory T cells, which track down and destroy infected cells and alert the B cells that it's time to start making new antibodies.

A study published in August analyzed the blood of 206 people in Sweden who had COVID-19 with varying degrees of severity. The researchers found that, regardless of whether a person had recovered from a mild or severe case, they still developed a robust T-cell response. In general, memory T cells can last for years.

But it's still unclear how long these T cells will remember the coronavirus, how effective their response is, or how much protection they can offer any given person (like President Trump). Much more research is needed.

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