Even when U.S. President Donald Trump claims that his fight with COVID-19 has granted him immunity, reports are emerging of people suffering from the disease for the second time. Although reinfection still seems rare, it is unclear to what extent immunity can truly protect a person.
Immunity is also in the news because there is a debate among scientists about herd immunity, the point at which enough people are immune to a pathogen to slow its spread (SN: 24/03/20). Although herd immunity may put the end of the pandemic in plain sight, experts estimate that about 40 to 60 percent of the population would have to be infected to reach it.
A group of researchers is pushing governments to achieve vaccine-free herd immunity, by allowing COVID-19 to spread among low-risk people by protecting vulnerable populations. However, this approach puts the entire population at risk of major illness and death, another group argues in a letter published Oct. 14 in the Lancet.
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Because SARS-CoV-2 is a new virus, scientists cannot say how long a person will be protected after recovering from an infection. If immunity decreases rapidly, this sets the stage for recurrent outbreaks unless there is a vaccine, the authors say.
Here’s what we know so far about our long-term defenses against coronavirus:
What does "immunity" really mean?
For scientists, immunity means resistance to a disease obtained by exposing the immune system to it, either through infection or through vaccination. But immunity does not always mean complete protection against the virus.
How does the body build immunity?
The immune system has two ways to provide lasting protection: T cells that remember the pathogen and trigger a rapid response, and B cells that produce antibodies, proteins the body makes to fight a specific pathogen.
Ideally, long after a person recovers from an infection, these antibodies remain in the blood. So, if the person is exposed to the same pathogen again in the future, those antibodies recognize the threat and work to prevent another infection from taking hold.
The so-called “memory T cells” are also around. Ideally, they fulfill their name and recognize a previously found pathogen and help coordinate the immune system or kill infected cells.
With one type of immunity, called sterilizing immunity, the virus never has a chance to begin to replicate and never infects a cell. However, sterilizing immunity is difficult to achieve. More often, people get partial immunity, which provides a quick response that can make the second attack of the disease less severe or less easily transmitted to other people.
It is not clear what kind of immunity people who have recovered from COVID-19 have and only time will tell. A vaccine could trigger a stronger immune response than a natural infection, although it remains to be seen whether that will be the case for coronavirus vaccines in trials (SN: 7/10/20).
If a person has antibodies, are they immune?
For some diseases, such as measles, antibodies can last a lifetime. But for SARS-CoV-2, the jury is still out. It is unknown how long the antibodies last in the blood or, more importantly, whether their presence is a sign of immunity. The fact that a person has antibodies does not mean that they are effective in fighting the virus.
Neutralizing antibodies are the ones that stop the virus, stopping infecting a host cell and replicating. These antibodies typically recognize the virus’s ear protein, which helps it break down into host cells. Until now, these types of antibodies have been the focus of studies that sought to understand whether a person can be immune.
“For most people, it looks like they’re generating neutralizing antibodies,” says Aubree Gordon, an epidemiologist at the University of Michigan in Ann Arbor. "So that's promising."
However, it is still unknown how much neutralizing antibody is sufficient for protection. And even though they are protective, it is unclear how long these immune proteins stay. Studies of patients recovered with COVID-19 have shown that antibodies to coronavirus may decrease after SARS-CoV-2 infection, but overall, their levels remain relatively stable over a period of three to six months.
Because the coronavirus only began its worldwide sweep earlier this year, "there was only a limited amount of time for people to study this," Gordon says.
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Some data suggest that the immune system may not have a good memory for coronavirus infections. One study found that during a COVID-19 infection, the organ that produces memory B cells – long-lived cells that will produce antibodies quickly if a person becomes exposed to a pathogen again – does not properly activate cell types capable of becoming B cells. memory. Without that immune memory, antibodies to SARS-CoV-2 may not last long, researchers reported Aug. 19 in Cell.
“Maybe that means those responses to memory are going to be on the short side,” says Brianne Barker, an immunologist at Drew University in Madison, N.J.
What do we know about T cells?
Studies have shown that patients with COVID-19 typically develop an immune response involving T cells. Even patients recovered without a detectable antibody response have T cells in their blood, researchers reported Oct. 15 in Emerging Infectious Diseases.
But the role of T cells in infection and immune memory remains unclear. Studies have shown that memory T cells can persist in patients infected with the coronavirus responsible for the 2003-2004 SARS outbreak for up to 11 years after recovery. However, since that virus no longer circulates, it is impossible to say whether those T cells can be protective.
It is possible that some people already have T cells that can recognize pieces of the new coronavirus (SN: 15/05/20). Those immune cells may be left over from previous coronavirus exposures that cause the common cold, researchers reported on Aug. 4 in Science. These cross-reactive T cells may help reduce the duration or severity of COVID-19 disease. On the other hand, these T cells can make the disease worse, perhaps by overstimulating the immune system and causing a disease called a cytokine storm, which is behind some severe cases of COVID-19.
Can you get coronavirus twice?
Researchers have now documented a small number of cases in which people became infected with the coronavirus twice. The first such case was reported in Hong Kong, with additional reports in the United States, the Netherlands and elsewhere (SN: 24/08/20).
But it is not yet clear what reinfections are like. And with just a handful of cases so far, “we can’t really say that reinfections are telling us much right now,” Barker says, whether it’s about immunity or whether vaccines will provide long-term protection or have to be part of our annual routine, such as flu vaccines.
Some reinfections are expected; some people’s immune memory may not be powerful enough to prevent infection altogether, although it can prevent them from getting sick.
It’s hard to prove that someone was re-infected, because researchers need to conclusively prove that two different viruses caused each infection, Barker says. That requires genetic testing. What’s more, experts aren’t necessarily looking for these cases, especially in people who don’t show symptoms.
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What does all this mean for the herd’s immunity?
Without knowing how long immunity lasts after an infection and how much it varies from person to person, it is impossible to know if ending the pandemic through herd immunity is even possible. What is clear, experts say, is that trying to achieve vaccine-free immunity of the herd will cause more disease and death.
"Promoting the concept of 'herd immunity' … in response to the COVID-19 pandemic is inappropriate, irresponsible and misinformed," Thomas File Jr., president of the Infectious Diseases Society of America, and Judith Feinberg, president of The Association of HIV Drugs, said in a statement on October 14.
And to date, immunity to command is still a long way off. “Across the United States, we’re not close to herd immunity,” Gordon says. "Taking the herd immunity approach through natural infection will lead to hundreds of thousands of unnecessary deaths."