New University of Minnesota research is contesting a key argument against COVID-19 vaccination — that people with prior coronavirus infections don't need further immunization to protect themselves.

Comparing blood samples following COVID-19 vaccinations in 48 participants, the U researchers found that everyone gained key memory B cells capable of producing antibodies that fight off the coronavirus, but people with previous infections gained five times more of those cells.

And in the world of immunology, more is better, said Marc Jenkins, a co-author and director of the Center for Immunology at the U Medical School. "There are some people who maybe had a prior infection and think, 'Well, I'm good to go,' and they certainly have some immunity. But this study shows if they complete the vaccination series they have a lot of immune potential."

The finding, published in the journal Cell Reports, comes as health officials try to increase Minnesota's vaccination rate and blunt a fourth wave in the pandemic that has caused 8,170 COVID-19 deaths and 714,790 coronavirus infections. That includes 17 deaths and 3,714 infections reported Friday.

The state also reported 799 COVID-19 hospitalizations on Thursday in Minnesota, where a 95% occupancy rate in intensive care units because of the pandemic and other medical issues has made it difficult to transfer patients to inpatient beds when needed.

Minnesota ranks 21st among states with a first-dose vaccination rate of 74.8% of eligible people 12 and older, according to the Centers for Disease Control and Prevention.

Gov. Tim Walz on Friday announced that vaccination clinics before the Minnesota Vikings games on Sunday and Oct. 10 will enter adult recipients in a lottery for free tickets to a future game. On Wednesday, Walz announced a clinic at the Minneapolis Convention Center will give recipients discounted access to the Twin Cities Con comic festival.

"The single greatest tool we have in this fight is the vaccine," Walz said.

Research this summer showed monetary incentives could motivate one-third of unvaccinated individuals to seek shots against COVID-19, but others would remain unconvinced. Risk of potential side effects remains the overriding concern in surveys of unvaccinated people, but confidence in the duration of natural immunity from previous infection is common as well.

While the immune system fights infection in many ways, the U researchers focused on the production of key memory B cells that are specific to the coronavirus and produce more antibodies in the event of a reinfection.

"They're like the insurance policy for the future infection," Jenkins said.

Importantly, they also work even if the virus mutates or produces new variant strains. The U researchers found that about 75% of the B cells generated in response to the original coronavirus strain started producing antibodies that bound to the beta variant.

The timing of the research prevented testing for the response to the fast-spreading delta variant, which largely caused the latest wave and is involved in more than 90% of new infections in Minnesota.

Jenkins said a similar response against the delta variant is likely, but that this small U study needs follow-up research to confirm that and to look for any demographic patterns in who develops the most memory B cells.

The research raised some questions about the timing of the U.S. vaccine schedule, which recommends two doses of the Pfizer vaccine three weeks apart, and two doses of the Moderna vaccine four weeks apart.

The study didn't assess B cell production in response to the single-dose Johnson & Johnson vaccine.

People tended to gain the majority of growth in memory B cells after two exposures of any kind — meaning either prior infection followed by a first dose, or by two doses of vaccine, the study found.

A third scheduled dose in people with previous infections wasn't as effective, because it came so soon that antibodies cleared it out before it could produce more memory B cells.

Jenkins said those findings suggest that people with prior infections might need only a single dose — sparing supplies for others — but such a change in the vaccine schedule would prove complicated. The findings also suggest a second vaccine dose is probably more effective when provided later — once antibodies produced in response to the first dose have cleared out.

The U.S. vaccine schedule was developed largely in urgent response to the pandemic — wanting to produce doses quickly rather than wait for research to prove that a delayed delivery schedule would be more effective.

"If we could go back in the time machine, that would have been a better strategy — first shot and then wait two months for the second shot," Jenkins said. "But even with only a one-month lag, it has worked really well."

U researchers were involved in development of an alternative COVID-19 vaccine approach — stimulating the immune system's T cells that are more blunt in their approach and would work against variants. However, Jenkins said that work has slowed because current vaccines have been so effective — and researchers instead are looking at influenza as a target.

Jeremy Olson • 612-673-7744