Nearly 30% of corona­virus infections in the first four weeks of August have involved people who were fully vaccinated, according to Minnesota breakthrough COVID-19 data released Monday.

State health officials said a higher proportion of breakthrough infections was inevitable as more people received shots and the coronavirus had fewer unvaccinated people left to target. But they stressed that the vaccine continues to provide strong protection against severe COVID-19.

"The vaccines are still highly effective at preventing serious illness, hospitalizations and deaths," said Doug Schultz, a spokesman for the state health department.

Minnesota typically analyzes breakthrough infections in comparison with fully vaccinated individuals — with the latest total of 12,559 breakthrough infections representing 0.42% of more than 3 million fully vaccinated people.

That is an increase from early July, when the breakthrough infections equated to 0.11% of the state's fully vaccinated population. However, the 810 people hospitalized with breakthrough infections represent only 0.027% of fully vaccinated people, and many were admitted for other reasons and discovered infections only upon routine screening. Eighty COVID-19 deaths have involved breakthrough infections, or 0.002% of fully vaccinated Minnesotans.

State leaders internally have started looking at breakthrough data in a secondary way — by comparing total infections with breakthrough infections in the same week.

Fewer than 10% of new coronavirus infections occurred in fully vaccinated people in the weeks before June, but the rate increased this summer. In the first four weeks of August, roughly 30% of infections were in fully vaccinated individuals — with data for the two most recent weeks being preliminary because of the time lag in verifying breakthrough cases.

Schultz said the data doesn't offer strong conclusions about vaccine effectiveness or why breakthrough infections are starting to make up more of the total infections each week.

It is possible that the increase reflects the infectiousness of the delta variant of the coronavirus, which is causing more than 90% of new infections in Minnesota. The increase also could suggest some waning in vaccine effectiveness over time, but Schultz said there is no pattern in the breakthrough infections showing earlier recipients at greater risk.

"We aren't seeing a huge spike in cases who are more than four months out from vaccine," he said in the statement.

Overall COVID-19 numbers are rising as well in Minnesota, where 591 people with COVID-19 were hospitalized on Friday.

The addition on Monday of six COVID-19 deaths and 1,918 coronavirus infections raised Minnesota's totals in the pandemic to 7,805 deaths and 646,094 infections.

Immunization levels have improved ahead of the start of the school year — with 70.3% of eligible people 12 and older receiving at least first doses of COVID-19 vaccine in Minnesota.

"COVID-19 vaccines are the best tools we have to keep our children and families safe and beat this pandemic," Gov. Tim Walz said Monday, commending the YMCA of the North for scheduling a series of public vaccine clinics.

Minnesota's breakthrough infection trends match with multiple studies.

Researchers at several U.S. sites, including St. Luke's in Duluth, have tracked COVID-19 vaccine effectiveness in hospital workers and first-responders who received some of the first doses after they became available in December 2020. They reported last week that vaccine effectiveness at preventing infection remained strong at 80% but had declined from 91% to 66% with the emergence of the delta variant.

Mayo Clinic researchers reached similar findings earlier this month about waning protection from infection, but they also found that the two-dose Moderna and Pfizer vaccines were strongly protective against severe illness and hospitalization — which is what federal regulators approved them for in the first place.

Federal health officials are planning to make booster doses of the Pfizer and Moderna vaccines widely available as early as Sept. 20.