Mayo Clinic researchers are reporting a "pronounced reduction" this summer in the effectiveness of the Pfizer COVID-19 vaccine at preventing coronavirus infections, but still a high level of protection against severe illnesses and hospitalizations.

Examining records for 25,000 vaccinated and unvaccinated Mayo patients in Minnesota, the researchers reported 76% effectiveness in the Pfizer vaccine protecting them from infection this year, but only 42% effectiveness in July amid the emergence of a more transmissible delta variant. The comparable effectiveness of the Moderna vaccine declined from 86% to 76%.

The study authors remained bullish about COVID-19 vaccines, noting that the Pfizer version's effectiveness against hospitalizations was 75% in July, and that the Moderna vaccine's rate was 81%. The two-dose vaccines received federal emergency authorization in December based on their abilities to prevent severe illnesses; initial trials hadn't even explored whether they also prevented mild or asymptomatic infections.

"This study further supports the effectiveness of both vaccines … despite the evolution of more transmissible viral variants," said the study, co-authored by COVID-19 experts at Mayo and nference, a data partner in Massachusetts.

The effectiveness of the Pfizer and Moderna vaccines at preventing hospitalizations was 85% and 91%, respectively, during the entire year, so their performance in July alone was slightly worse.

The study did not assess the effectiveness of the single-dose Johnson & Johnson vaccine, which was approved later and has been used less in Minnesota. The state on Thursday reported that more than 6 million COVID-19 vaccine doses have been administered to more than 3.2 million eligible people 12 and older.

Antranik Tavitian, Star Tribune, Star Tribune

Pfizer's vaccine made up 57% of the doses administered. Johnson & Johnson's version made up 4.5%.

The study posted Monday on medRxiv has not undergone peer review. News of its findings emerged Thursday as Minnesota leaders celebrated a vaccination milestone — more than 70% of people 16 and older have received at least first doses in the state.

Gov. Tim Walz set that goal in May — before the age range for the Pfizer vaccine was reduced from 16 to 12 — with the hopes of Minnesota reaching it by July 1. Vaccination numbers have strengthened in recent weeks amid concerns about the delta variant and a new $100 incentive program.

"While we are making progress, there is no time to waste in making sure every Minnesotan who can get the shot does so," Walz said. "It will save lives."

The delta variant has increased COVID-19 levels in Minnesota, where the positivity rate of diagnostic testing has risen from 1.1% to 5.1% over the past month and hospitalizations have increased from 90 to 377.

The Minnesota Department of Health on Thursday reported eight more COVID-19 deaths and 1,318 more coronavirus infections, raising totals in the state to 7,723 deaths and 623,527 infections.

Breakthrough coronavirus infections in fully vaccinated people are rare, but appear more likely when the delta variant is circulating. Minnesota has found 5,599 breakthrough infections in 3 million fully vaccinated individuals — a rate of 0.19%.

Mayo researchers were not available for comment Thursday evening, but their study said breakthrough infections are inevitable and that the COVID-19 vaccines outperform others. The effectiveness of seasonal influenza vaccines at preventing infections has ranged from 19% to 60% over the past decade.

The authors said more research is needed in other parts of the world to verify the findings in Minnesota. The study did not assess whether infections were more common in people who had received earlier vaccinations. The findings could point to the need for booster vaccine doses to maintain protection.

State infectious disease director Kris Ehresmann on Friday said the Mayo findings are important but that more vaccinations are needed "because really the breakthrough infections that we're seeing are the collateral damage to vaccinated persons from what's happening predominantly in our unvaccinated community."

"While it's important to know how variants change the landscape and how they are more transmissible and all of those things," she added, "I think the important message is that we need to get a greater proportion of our population vaccinated in order to be in a better place to control and address variants, or we will see more and worse things coming down the road."

Jeremy Olson • 612-673-7744