How Do We Become Immune?
When germs enter your body, your immune system springs into action. Here's how it works:
Bacteria and viruses like the one that causes COVID-19 have proteins called antigens on their surfaces. Each type of germ has its own unique antigen.
White blood cells of your immune system make proteins called antibodies to fight the antigen. Antibodies attach to antigens the way a key fits into a lock, and they destroy the invading germ.
Once you've been exposed to a virus, your body makes memory cells. If you're exposed to that same virus again, these cells recognize it. They tell your immune system to make antibodies against it.
Vaccines work in much the same way. They expose your body to an antigen that trains your immune system to fight that germ in the future. Because vaccines contain weakened or killed versions of viruses, you become immune without getting sick.
If You've Had COVID-19, Are You Immune?
Health experts don't know whether we really become immune to COVID-19 after we're infected. And if we do have immunity, we don't know how long it might last.
Other types of coronaviruses appear to trigger some immunity. Studies show that people are protected against the coronaviruses that cause the common cold for up to a year after an infection. And our bodies have antibodies against the virus that causes severe acute respiratory syndrome (SARS) for up to 4 years.
Most people who've recovered from COVID-19 have antibodies against the virus. But there's no evidence that this will protect them if they're exposed to it again.
A study in the United Kingdom found that most people who had COVID-19 lost their antibodies within a few months. Research in the United States on people with mild infections also discovered a decline, with COVID-19 antibodies lasting for a shorter time than those for the related coronavirus that causes SARS.
In South Korea, more than 160 people tested positive for the virus again after they had recovered. In China, 5%-10% of people tested positive again after they'd recovered, according to news reports. It's not clear whether:
These people got infected again
The virus reactivated in their bodies after being quiet for a while, or
The test results were flawed
Researchers think the antibodies in convalescent plasma (the liquid part of blood) from people who’ve recovered from COVID-19 might help people who are sick with the disease. If you’ve had a positive antibody test and want to donate plasma, visit your local blood donation center, or check out the National COVID-19 Convalescent Plasma Project (https://ccpp19.org/).
Could Herd Immunity Protect Us?
Herd immunity happens when a large part of the population -- the herd -- is immune to a virus. This can happen either because these people got vaccinated or had already been infected. Herd immunity makes it harder for a virus to spread. So even those who haven't been sick or vaccinated have some protection.
The more contagious a virus is, the more people need to be immune for herd immunity to kick in. The SARS-CoV-2 virus is so contagious that experts estimate about 70% of people in a community will need to be immune to have herd protection. That number might be hard to get to without a vaccine or a whole lot of people getting sick.
How Do we Test for Immunity?
Antibody tests, also called serology tests, measure antibodies to coronavirus in the blood. If you have antibodies, it means you've been exposed to the virus and your immune system has made antibodies against it. Antibody tests are different from the tests doctors use to check for the virus itself.
Because COVID-19 is so new, there hasn't been much time for scientists to check the accuracy of antibody tests. They could have false-positive results. That's when someone tests positive for antibodies but hasn't really developed them.
Testing for antibodies too soon after an illness can also cause false results. It takes 5-10 days after you get infected to develop antibodies against the SARS-CoV-2 virus.
Antibody tests could give people a false sense of security. They might go back to work and start to travel again when they could still catch or spread the virus. And because people can pass COVID-19 to others without showing symptoms, false positive results could lead to more outbreaks of the virus.