How Does Immunity Protect Against Viruses?

You may have read about tests for antibodies to COVID-19. But antibodies are just one part of the immune system. Here’s what you should know about immunity to all viruses.

We all want immunity from COVID-19. The vaccine for coronavirus began rolling out to meet the demand in December. Some people who suspect they’ve had the illness are taking blood tests to see if they carry the immune molecules called antibodies.

But it’s important to know that antibodies are just one part of your immune system, and they typically drop off after an acute infection. Meanwhile, other kinds of immune cells called memory T cells linger — in fact, with some coronaviruses they remain to protect you for a lifetime. Most vaccines under development will be designed to trigger a kind of memory T cell.

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Why does immunity matter?

Let’s say you were ill with clear symptoms that doctors recognized as COVID-19 or symptoms confirmed by a positive test for infection. That illness would normally give you some protection from getting sick again. Now, let’s say you take an antibody test months later. If it comes up negative, you might panic that you never actually had the virus or somehow didn’t develop any immunity.

Some people might argue that those cases suggest vaccines might not work. After all, vaccines work by triggering an immune reaction you’d normally get if you fell ill.

The truth is that those fears are based on misunderstandings of normal immunity. We don’t know exactly how immunity to COVID-19 works, but so far we have no evidence that it’s very different from other coronaviruses.

The accuracy of antibody tests for COVID-19 is still under investigation. There have been many false-positive results. Such results can mean that someone who was tested positive for the virus may not have had time to develop antibodies to the virus, especially if they’ve been tested soon after contracting COVID. In fact, it can take 5 to 10 days after you are infected to develop antibodies.

So how does immunity to COVID-19 actually work?

Scientists are putting the picture together now. We know that people infected with the virus do typically produce antibodies, as they would with another virus. For example, in a study of almost 20,000 infected individuals with mild-to-moderate cases screened at Mount Sinai Health System in New York City most patients produced antibodies that lasted for three months.

That’s predictable. During an infection, cells called plasmablasts expand into a pool of millions and secrete antibodies, which block the virus from infecting cells. This big effort can’t last forever. Eventually, most plasmablasts die. A small number enter bone marrow to wait for an occasion when you may need those precise antibodies again.

Scientists also have evidence that the immune response to the coronavirus follows that path, triggering memory T cells. A team at the La Jolla Institute for Immunology in California found a particular kind of memory T cell in all sick patients, and the sicker patients had more of them, a sign of a strong response to a strong threat.

With other coronaviruses, we seem to be most protected for a bit less than a year, and then immunity declines over the next two years. However, the chances are that if you do get sick again, the symptoms will be milder.

In a recent case in Hong Kong, for example, a man with mild COVID-19 symptoms during his first round was infected again about four and a half months later. The second time he had no symptoms at all, learning about the new infection only because it showed up in a sample at the airport after he returned from a trip to Europe. Could the second positive test simply be detecting the old virus? Scientists said no because the coronavirus was slightly different, reflecting cases in Europe rather than Hong Kong.

In Reno, Nevada, a man also seemed to have had infections with clearly distinct versions of the virus, but his second infection caused more serious symptoms. Scientists hope that his experience will prove rare, but we don’t yet know.

Some level of immunity may be widespread. The coronavirus that causes COVID-19 is new to our bodies, but we’ve all been exposed to other coronaviruses, which include many of the pathogens that cause the common cold. Other research has found that many people have memory T cells that recognize a protein that looks similar in all coronaviruses, which suggests the cells could blunt the symptoms of the virus. This may explain why some people infected with COVID-19 have mild symptoms or none.

Because people’s immune systems vary, these individual stories can’t give you any certainty about your own risks. The bottom line: You could flunk your antibody test and still have had COVID-19 and be protected from a second infection, at least for a while. Nonetheless, we all need to play it safe. Even if you’re sure you’ve had the virus, your immunity will eventually wear off, just like it does with any flu and why you need a flu shot every year. Like everyone else, you need to avoid infection and get vaccinated as soon as you can.

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