Researchers and clinicians around the world are working at an expedited pace to answer many questions related to COVID-19—one of the most pressing is the question of immunity. In the short term, understanding the immunity spectrum, and where those who’ve recovered from the virus fall within it, could inform medical professionals of a potential end of the community spread of the virus as well as aid the development of policy and large-scale decision making.

Ultimately, unless a vaccine precedes it, confirmation of immunity will help determine how quickly society can recover from the pandemic and return to normal life.

Andrew Luster, MD, PhD, chief of the Division of Rheumatology, Allergy and Immunology at Massachusetts General Hospital, shares more about how immunity is tested and what it could mean in the case of COVID-19.

Q. How do we test for immunity?

A. The standard way of testing for immunity to a virus is to look for antibodies to the virus in the blood following infection—proteins that the immune system produces to defend against pathogens, such as bacteria and viruses. In contrast, the nasopharyngeal (nose and throat) cotton swab tests, often featured in the media, are diagnostic tests that confirm whether an individual has an active infection by searching for the genetic material of SARS-CoV-2, the novel coronavirus that causes COVID-19.

Q. What are antibodies?

A. There are several types of antibodies produced in response to an infection. The earliest antibody produced in a primary response is immunoglobulin M (IgM) that begins to appear five to seven days after infection. The presence of IgM antibodies specific for a pathogen indicates an acute infection because these antibodies rapidly decrease over time after infection and are replaced by immunoglobulin G (IgG) antibodies.

If someone has immunity, and they have already cleared the virus, they will have IgG antibodies but not a lot of IgM antibodies. When someone typically becomes reinfected with the same strain of the virus, the so-called secondary response is dominated IgG antibodies that remain in the circulation to provide protection. Memory B cells that make these IgG antibodies are quickly activated and produce antibodies more rapidly in the secondary response compared to the primary response.

Since COVID-19 is a respiratory illness, additional types of antibodies could also be important in developing immunity. For instance, immunoglobulin A (IgA) antibodies, which are antibodies secreted into mucosal surfaces, like in the gut and the lung, could ultimately be important for immunity to a respiratory pathogen like SARS-CoV-2.

Q. Can antibody tests prove COVID-19 immunity?

A. It is essential to find out exactly how our immune systems are fighting the novel coronavirus. As our knowledge grows, we'll be able to tell exactly which antibodies are able to neutralize the virus and are thus protective.

We also don't yet know how long immunity to the novel coronavirus could last. In some cases, such as chicken pox, after you recover from the illness, you are essentially immune forever. However, in other infections or immunizations, immunity does not last forever. For example, immunity to the coronaviruses that cause the common cold seem to only last two to three years. We don’t know yet how long immunity to SARS-CoV-2 will last and if the degree of immunity will differ depending on the severity of an individual’s illness. These will be very important questions to answer in the future to understand the risks of reinfection.

Q. Is immunity the key to ending physical distancing restrictions?

A. Serology tests—the blood tests that identify antibodies—could be the key to reopening schools and to allowing people to reenter their workplaces. However, our immune systems are facing a brand new enemy, so we need to proceed with caution as there are many questions still unanswered, including the permanence of the immunity and/or if there is potential for those with positive antibodies to still be carriers of the virus.

In April, the Food and Drug Administration (FDA) approved the country's first test for coronavirus antibodies. Given the current circumstances, the normal checks and balances for science are being expedited. Research is being disseminated before peer-review and tests approved without the usual FDA review. The scientific community has been uniting in an unprecedented way to share research faster than ever before. This is good because it gives scientists a chance to collaborate, confirm and expand on each other's research more quickly, but it can also be tricky if preliminary research that has not been properly vetted is influencing policy.

As we press on for a deeper understanding of COVID-19 immunity, supporting a healthy immune system is a tactic well within control. Eat a well-balanced diet, exercise, wash your hands and maintain physical distance from people outside of your home.

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