A bout of COVID-19 infection may not be enough for long-lasting immunity as the virus continues to mutate and new variants emerge.
Infection-acquired immunity from catching COVID-19 can act as a protective layer for a period of time, but that protection wanes just as vaccine-induced immunity does, experts say, noting infection from one variant may not provide any protection against another.
In Canada and globally, the Omicron sub-variant BA.5 is dominant, but the World Health Organization (WHO) is tracking hundreds of others.
Since its emergence in late 2019, COVID-19 has produced multiple mutations with Alpha, Beta, Gamma, Delta and Omicron so far characterized as variants of concern.
COVID-19 infection does not always result in a good immune response, experts say — and reinfections can happen.
So, how long are you immune to COVID-19 after infection?
Just like with vaccine-induced immunity, by the sixth month, there is “literally no protection left” from infection-acquired immunity, said Dr. Catherine Hankins, a professor in the School of Population and Global Health at McGill University, and co-chair of Canada’s COVID-19 Immunity Task Force.
The type of variant one is infected with also plays a role in so-called natural immunity.
For example, if you were infected with the Delta variant and then got exposed to Omicron, you would not be protected against Omicron, although protection against severe disease is likely still intact, Adalja said.
That said, there is going to be some level of protection, lasting a period of two months, where you are not likely to be infected with another Omicron sub-variant if you were initially infected with another sublineage of Omicron, Adalja said.
“It all depends upon how far that variant is from whatever version of the virus you got infected from.”
What does the science show?
There is no guarantee that infection will induce immunity, according to a review by the COVID-19 Immunity Task Force.
A Canadian study published in the Pediatric Infectious Disease Journal in May found that one in every eight people who contract COVID-19 does not develop antibodies in their blood from the illness. And children are half as likely to develop immunity from an infection, according to the CITF-funded research.
Meanwhile, there is also growing data to suggest that getting infected with Omicron does little to protect you from catching the virus again.
A U.S. preprint study — which has not been peer-reviewed — released in January 2022 suggested that mild Omicron infection doesn’t render enough immunity to prevent future infections, while infections from the Delta variant, which tended to be more severe, produced higher protection.
According to the U.S. Centres for Disease Prevention and Control (CDC), COVID-19 antibodies peak within the first few weeks following symptom onset, then fall below detectable limits two to three months after infection.
Another U.S.-based study published in the New England Journal of Medicine in July showed that BA.4, and BA.5 subvariants of Omicron substantially escape neutralizing antibodies induced by both vaccination and infection.
A paper out of Qatar, the results of which were made available online last week in a letter ahead of peer review, found that protection from a previous infection against BA.4 or BA.5 reinfection was modest when the previous infection had been caused by a pre-Omicron variant but strong when it had been caused by a post-Omicron subvariant (including BA.1 or BA.2).
More research is needed, Adalja said.
“It’s not one size fits all. It’s hard to actually predict because we don’t have a lot of data on these variants and who’s getting infected with this versus what they got infected with in the past.”
Hybrid immunity
Despite the antibodies that one can develop from COVID-19 infection, vaccination remains an important tool at our disposal — whether or not you get infected, experts say.
“The best form of protection, actually, is to get all of your vaccine doses,” said Hankins, adding that infection brings with it the risk of long COVID and other complications.
“It’s just too much of a risk to take for something that is vaccine-preventable,” she added.
Compared with natural immunity from a bout of COVID-19, vaccine-induced immunity might be more protective when it comes to certain variants, Adalja said.
This is where hybrid immunity comes in. According to the WHO’s definition, that is immune protection in individuals who have had COVID-19 at least once and received one or more vaccine doses.
The Qatari study showed that hybrid immunity with two or three doses of an mRNA vaccine and a prior infection was associated with the highest degree of protection against symptomatic infection.
“Hybrid immunity seems to be the strongest at protecting against severe disease, hospitalization and death,” Adalja said.
“It adds to and augments the natural immunity you get from infection.”