What if you had COVID and later got vaccinated?
This is called hybrid immunity, and it’s the best of both worlds.
“You have the benefit of very deep, but narrow, immunity produced by vaccine, and very broad, but not very deep, immunity produced by infection,” Dr. Poland says. He says you’ve effectively cross-trained your immune system.
In studies of people who recovered from COVID-19 and then went on to get an mRNA vaccine, after one dose, their antibodies were as high as someone who had been fully vaccinated. After two doses, their antibody levels were about double the average levels seen in someone who’d only been vaccinated.
Studies have shown this kind of immunity has real benefits, too. A recent study by researchers at the University of Kentucky and the CDC found that people who’d gotten COVID-19 in 2020, but had not been vaccinated, were about twice as likely to be reinfected in May and June compared with those who recovered and went on to get their vaccines.
What antibody level is protective?
Scientists aren’t exactly sure how high antibody levels need to be for protection, or even which kinds of antibodies or other immune components matter most yet.
But vaccines appear to generate higher antibody levels than infections do. In a recent study published in the journal Science , Dr. Weiskopf and her colleagues at the La Jolla Institute of Immunology detail the findings of a de-escalation study, where they gave people one-quarter of the normal dose of the Moderna mRNA vaccine and then collected blood samples over time to study their immune responses.
Their immune responses were scaled down with the dose.
“We saw that this has the exact same levels as natural infection,” Dr. Weiskopf says. “People who are vaccinated have much higher immune memory than people who are naturally infected,” she says.
Antibody levels are not easy to determine in the real world. Can you take a test to find out how protected you are? The answer is no, because we don’t yet know what antibody level, or even which kind of antibodies, correlate with protection.
Also, there are many different kinds of antibody tests and they all use a slightly different scale, so there’s no broadly agreed upon way to measure them yet. It’s difficult to compare levels test to test.
Weeks or months between doses? Which is best?
Both the Pfizer and Moderna vaccines were tested to be given 3 and 4 weeks apart, respectively. But when the vaccines were first rolling out, shortages prompted some countries to stretch the interval between doses to 4 or more months.
Researchers who have studied the immune responses of people who were inoculated on an extended dosing schedule noticed something interesting: When the interval was stretched, people had better antibody responses. In fact, their antibody responses looked like the sky-high levels people got with hybrid immunity.
Susanna Dunachie, PhD, a global research professor at the University of Oxford in the United Kingdom, wondered why. She’s leading a team of researchers who are doing detailed studies of the immune responses of health care workers after their vaccinations.
“We found that B cells, which are the cells that make antibodies to the viral spike protein after vaccination, carry on increasing in number between 4 and 10 weeks after vaccination,” she says.
Waiting to give the second vaccine 6 to 14 weeks seems to stimulate the immune system when all of its antibody-making factories are finally up and running.
For this reason, giving the second dose at 3 weeks, she says, might be premature.
But there’s a tradeoff involved in waiting. If there are high levels of the virus circulating in a community, you want to get people fully vaccinated as quickly as possible to maximize their protection in the shortest window of time, which is what we decided to do in the United States.
Researchers say it might be a good idea to revisit the dosing interval when it’s less risky to try it.
A version of this article first appeared on WebMD.com.