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Whether or not someone has recovered from COVID-19, conferring some level of immunity, is crucial to determine when they can safely get back to work and out in public again.
As a result, how to test for COVID-19 immunity on a massive scale is a central question for local leaders in the U.S. as they plan to reopen the country.
But how does coronavirus immunity work?
If someone is infected with the novel coronavirus, do they become immune? If so, does it always work? If you become immune, how long will it last?
The answers to these questions could determine how long the coronavirus lockdown lasts.
How Immunity Works
There are three ways a person can be immune to a disease, according to MedicineNet medical author William C. Shiel Jr., MD, FACP, FACR.
First, he says, people can be innately immune, as they are for example to the dog disease canine distemper. This virus causes great harm to dogs and puppies, and spreads easily to other dogs, but human beings cannot be infected with it.
The next way a person can become immune, says Dr. Shiel, is by developing an immune response after you have already been infected. Your body produces specialized white blood cells that adapt to new infections in order to destroy them, according to Medscape medical author Pedro A. de Alarcon, MD.
This is called adaptive immunity, and it can protect you for a certain time against a given virus, but the protection lasts longer for some diseases than others.
The duration of a vaccine's effectiveness can vary due to the nature of the infection, but also due to how the vaccine is prepared. Live vaccines usually provide longer-lasting protection than some other vaccines, according to New Zealand's Immunisation Advisory Centre.
What We Know About COVID-19 and Immunity
We know very little for certain about the novel coronavirus COVID-19, although studies of other coronaviruses suggest how it might work.
In the late 1970s, doctors inoculated 18 volunteers with a common, mild coronavirus that causes cold symptoms, as described in the New York Times by Dr. Marc Lipsitch. The volunteers developed colds as expected. To see how their immune systems would protect them, a year later 6 of the volunteers were inoculated again with the same virus. None of them became infected again.
Researchers inoculated the remaining 12 volunteers to a slightly different coronavirus strain, but they showed only partial immune protection from that virus.
We have not run these tests on the more dangerous known coronaviruses SARS or MERS, mostly because those deadly infections have not infected as many people, Dr. Lipsitch said. But we have tested the antibodies produced in the blood of people infected with these diseases.
The antibody tests revealed that immunity lasts two years against SARS infection, and nearly three against MERS, Dr. Lipsitch said.
"However, the neutralizing ability of these antibodies — a measure of how well they inhibit virus replication — was already declining during the study periods," he said.
These data have led researchers to suspect that people who recover from COVID-19 infection remain immune to the virus for a year or more. However, this hasn't been proven.
New studies have begun to emerge that shed light on how immunity may work against SARS CoV-2, the virus that causes COVID-19.
One fact has emerged regarding antibody testing. Antibodies—the white cells designed to fight a specific pathogen—take time to emerge in people infected by SARS CoV-2. In the first 10 days of infection, these tests cannot show that a person has the disease.
One study out of China found that it takes up to 15 days on average for some of the antibodies to emerge in an infected person's blood.
But what policymakers want to know is whether a new outbreak of COVID-19 is likely to be as dangerous as the current one. And they need to know how to prepare either way.
Why 'Herd Immunity' Is Needed for COVID-19
Dr. Lipsitch is an expert on the topic of coronavirus. He thinks it is likely that herd immunity will eventually develop around COVID-19, but that it hasn't happened yet.
Herd immunity is commonly understood as "the presence of immune individuals in a population (who) can indirectly protect those who are not immune against infection," says epidemiologist Caroline L. Trotter, Ph.D., for Medscape.
That means the more people in a population who have recovered from an infection, the more immune people there are in the population. And their immunity has the effect of protecting others who are not immune.
That is especially important for people who cannot produce their own immune response to a virus, and is one of the main reasons why the flu shot is recommended for all people each year.
When infections are spread, "transmission will be highest in a fully susceptible population," Dr. Trotter said. Such is the case right now across the world for COVID-19.
Dr. Lipsitch has worked on teams studying thousands of cases of seasonal coronavirus in the United States. From this work he concludes "immunity over a year or so is likely for the two seasonal coronaviruses most closely related to SARS-CoV-2 -- an indication perhaps of how immunity to SARS CoV-2 itself might also behave."
He explains that for the number of new infections to go down, more and more people need to become immune to the virus. When the average infected person spreads infection to less than one other person, this will cause the number of new cases to fall.
All of this means that immunity—and herd immunity specifically -- are the keys to leaving lockdown. Whether that immunity comes from natural infections over time, or instead through the development of a vaccine, only time can tell.