Could COVID-19 infection immunity substitute for vaccine? Don’t count on it, says expert

Employers who mandate the COVID-19 vaccine for applicants or employees would be required to waive that rule for unvaccinated people who have survived the illness under draft legislation that began circulating in the Capitol this week.

The two kinds of immunity — that conferred by catching the virus and that available through the vaccine — are not reliably the same, however, according to a University of Wisconsin epidemiologist, and there is no widely available test to demonstrate how well a person who has survived one infection is protected from being infected a second time.

“Natural infection does produce an immune response, but not all immune responses will be durable enough and heightened enough to ward off reinfection at some point,” said Ajay Sethi, faculty director for the  Master of Public Health program at the University of Wisconsin-Madison School of Medicine & Public Health. “So the question becomes, which source of immunity will provide more reliable protection — and vaccines afford that.”

On Monday, Rep. Cody Horlacher (R-Mukwonago) and Sen. Mary Felzkowski (R-Irma) began circulating a draft bill that would essentially consider vaccinated people and survivors of COVID-19 as equals where employer vaccine mandates are concerned.

The legislation applies to employers that require employees or job applicants to demonstrate proof of a COVID-19 vaccine or else take regular tests for the virus. In place of either vaccination or testing, it requires those employers to accept “documentation demonstrating natural immunity against the SARS-CoV-2 coronavirus” — either a previous, positive test for COVID-19, or a test showing the presence of “naturally occurring antibodies” against the virus.

The co-sponsorship memo doesn’t cite evidence that “natural immunity” is equivalent to the vaccine. Instead it focuses on health care or personal service workers who are leaving the workforce in response to vaccine mandates.

But in an interview on Tuesday, Sethi told the Wisconsin Examiner that a test that shows previous infection does not match vaccination when it comes to establishing a person’s immunity.

“The idea of looking for proof through a test that you’re protected is not reliable enough,” he said. “The best way of knowing whether the public is protected, and if individuals are protected, is vaccination status.”

While natural infection can confer immunity in some people, the immunity produced by the COVID-19 vaccine is more reliable, Sethi said. “That’s been shown through clinical trials and effectiveness data since the vaccines were put in use.”

Blood tests can detect antibodies for the virus, but they don’t measure immunity. “The serology tests that exist provide evidence of past infection, but they don’t provide evidence of protection moving forward,” he said.

To determine how well a person is protected from the virus requires a group of tests, measuring other immune system components besides antibodies. Taken together, they show more than just how strong the immune response is now; they offer evidence that it will last.

But such tests are done in the laboratory, not in the general population. “Those types of tests can’t be done at scale,” Sethi said.

Instead, public health providers rely on monitoring infection rates, hospitalization rates and deaths among vaccinated and unvaccinated people. For COVID-19, “those data are extremely revealing that vaccinations do afford protection against hospitalization and death,” Sethi said, “and quite a bit of protection against infections in general.”