Both types can play a role in future protection from—and even the potential treatment of—a coronavirus infection.

By Leah Groth
April 14, 2020
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Right now, more than 450,000 people worldwide have reportedly recovered from COVID-19, and that number keeps ticking up, according to Johns Hopkins University's COVID-19 dashboard. That's good news, of course; but just because someone has contracted and ultimately healed from the new coronavirus, doesn't mean they're entirely exempt from re-infection. That's where the issue of immunity comes into play—and currently, researchers and scientists still aren't sure what the body's immune response to SARS-CoV-2 is, and how long it might last.

For the most part, the word 'immunity' conjures up thoughts of complete protection from an illness, but it's much more complicated than that, according to the Centers for Disease Control and Prevention. Generally speaking, a person achieves immunity to a disease through the presence of antibodies, or proteins produced by the body that can neutralize or even destroy toxins or other disease carriers. "These are our 'attack mechanisms' against 'invaders,'" Jaimie Meyer, MD, a Yale Medicine infectious disease doctor and assistant professor of medicine at Yale School of Medicine, tells Health. Those antibodies are also disease-specific, per the CDC—it's why this year, for example, even if you got your flu shot, you have no immunity against the current coronavirus outbreak.

Past that, immunity itself can be broken down into two different categories: active immunity and passive immunity—and those differences depend on how the body was introduced to whatever virus or bacteria it developed antibodies for, and to what extent (and for how long) they can prevent future disease. The good news: Both types can play a role in future protection from (and even the potential treatment of) COVID-19.

What is active immunity and how is it acquired?

The CDC explains that active immunity "results when exposure to a disease organism triggers the immune system to produce antibodies to that disease," and can happen one of two ways: Through infection with the actual disease, which is known as natural immunity; or through a vaccination (essentially, a killed or weakened form of the disease that won't make someone ill, but will trigger the body to make antibodies), which is known as vaccine-induced immunity.

Active immunity that results from either situation—natural immunity or vaccine-induced immunity—will allow a person's immune system to recognize the specific disease, if they ever come into contact with it again, which will then trigger the body to produce the antibodies needed to fight it off.

Per the CDC, active immunity is often longer-lasting and may sometimes even provide life-long protection—but that's entirely based on the disease itself. Immunity to the varicella virus (aka, chickenpox)—either via acquiring the infection as a child or through a vaccine—can provide lifelong immunity or long-lasting protection for up to 10 to 20 years, according to the CDC. Whereas a yearly flu shot must be repeated annually, as it provides the most protection within the first three months, and begins to lose most effectiveness after six months.

It's also important to note that active immunity isn't immediate—it can sometimes take several weeks to develop, which is why most doctors, including the CDC, suggest getting your yearly flu shot by the end of October, in order to have protection by the time flu season begins to pick up in November or December.

While much more research needs to be done on immunity as it pertains to COVID-19, Dr. Meyers says both natural immunity and vaccine-induced immunity may provide protection against coronavirus—again, through infection by the virus or a forthcoming vaccine. And while vaccine-induced immunity is still a huge question mark—and will continue to be for at least another year until more trials are completed—researchers are currently looking at the immunity gained by those who have recovered from COVID-19.

During a World Health Organization media briefing on April 13, Michael Ryan, MD, MPH, executive director of the WHO's Health Emergencies Programme, shares that it's still "unknown" whether those previously infected with COVID-19 can be re-infected, and what type of immunity they have against the virus. "One would expect that a person who generates a full-blown immune response with detectable antibodies should have protection for a period of time," Dr. Ryan said. "We just don't know what that period of time is. We would expect that to be a reasonable period of protection, but it is very difficult to say that with a new virus."

Maria Van Kerkhove, PhD, an emerging diseases expert and technical lead on COVID-19 for the WHO, echoed this statement, after explaining that preliminary studies out of China offered mixed findings on immunity. "Right now, we don't have a full picture of what immunity looks like," Dr. Van Herkhove said. "And until we do, we can't give a complete answer."

Still, some experts—including the foremost COVID-19 resource in the US, Dr. Anthony Fauci, the government's top infectious disease specialist—feel confident that those exposed to and infected by the coronavirus will develop some immunity. "If this virus acts like every other virus that we know, once you get infected, get better, clear the virus, then you’ll have immunity that will protect you against re-infection,” Dr. Fauci said in a recent interview on The Daily Show.

What is passive immunity and how is it acquired?

While active immunity occurs when an individual produces antibodies to a disease through his or her own immune system, passive immunity is provided when a person is given antibodies. This can happen in utero or through antibody-containing blood products—such as immune globulin, or a substance made from human blood plasma—administered when immediate protection from a specific disease is needed. “For example, when a mother’s antibodies cross the placenta to the fetus or when people are given antibodies as treatment for rabies,” explains Dr. Meyer. Immune globulin can also provide protection against hepatitis A in instances when a hepatitis A vaccine is not recommended, per the CDC

The major advantage to passive immunity—and the reason why it's sometimes used as a treatment against diseases—is that it provides immediate protection. But passive immunity doesn't last as long as active immunity, and loses effectiveness within a few weeks or months, per the CDC.

Of course, this passive immunity may also be helpful when it comes to COVID-19—primarily through the potential use of convalescent serum or blood plasma collected from those who have previously recovered from COVID-19. This means, according to Dr. Meyer, "giving antibodies from the blood of people who have recovered from COVID-19 to people who are actively ill in order to prevent complications and hasten recovery." But the use of convalescent plasma isn't exactly new; it's also been used as a treatment option in a variety of other infectious diseases, including Ebola, Middle East respiratory syndrome (MERS), SARS, and even the H1N1 and H5N1 infections, according to research presented in JAMA. That same research found that, for five critically-ill patients with COVID-19, convalescent plasma treatment resulted in "an improvement in clinical status" in all patients, concluding that convalescent plasma may be a helpful treatment for those with critical cases of COVID-19.

Convalescent plasma as treatment for COVID-19 specifically is still being studied, and is not yet recommended as routine treatment—but while it's not yet been approved for use by the US Food & Drug Administration, the FDA did provide guidance to health care professionals and investigators administering or studying the use of convalescent plasma for treatment of COVID-19, according to a press release shared Monday, April 13. In addition to those guidelines. the FDA also approved Johns Hopkins University to test blood therapies for COVID-19 using plasma from recovering patients. "Researchers hope to use the technique to treat critically-ill COVID-19 patients and boost the immune systems of health care providers and first responders," researchers from Johns Hopkins University said in a press release.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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