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Delta variant drives big increase in breakthrough infections in Minnesota; health officials say trend shows importance of boosters

Despite their limitations, vaccines remain the best way to avoid a severe infection and to slow the spread of the coronavirus, health officials say.

Sanford Health began scheduling and administering the COVID-19 vaccine to children ages 5 to 11 on Thursday, Nov. 4. (Courtesy / Sanford Health)
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ST. PAUL — Minnesota is seeing dramatic growth in breakthrough cases of the coronavirus, adding to evidence that vaccines wear off over time and may be less effective against the now dominant delta strain.

In early May, fewer than 5 percent of new cases in the state were among fully vaccinated residents, according to data first released Friday by the state Department of Health. By early October, fully vaccinated people accounted for nearly 40 percent of new infections and similar shares of hospitalizations and deaths.

For health officials, the trends highlight the importance of booster shots, which soon may be available to nearly every adult, as well as continued preventive steps like social distancing and wearing masks in public.

“We want to make sure people are aware of the fact that the landscape has really changed since last spring,” said Kris Ehresmann, director of infectious disease prevention for the Minnesota Department of Health.

Ehresmann acknowledged she was among many public health officials who “naively” thought last spring that vaccines would control the pandemic. But vaccine skepticism and the emergence of variants changed that.


“We are seeing the impact of delta,” she said. “I think we are in a very different place right now.”

For the year in Minnesota, 16 percent of coronavirus infections, 15 percent of hospitalizations and 14 percent of deaths from COVID-19 have been among vaccinated residents, a Pioneer Press analysis found.

Despite their limitations, vaccines remain the best way to avoid a severe infection and to slow the spread of the coronavirus, health officials say.

“You are 15 times more likely to be hospitalized or to die of COVID if you are unvaccinated,” Ehresmann said.

What we know about breakthroughs

Minnesota has been tracking breakthroughs since January, soon after vaccinations began. The health department first reported them publicly Aug. 10 when there were only about 5,600 cases out of 2.9 million people who were fully vaccinated.

Since then, the number of breakthroughs has jumped to nearly 65,000 cases out of 3.2 million vaccinated residents. Of those, 2,956 have been hospitalized and 483 have died.

Health officials released weekly rates of vaccine breakthroughs for the first time Friday. While they show a considerable increase in not only breakthrough cases but also hospitalizations and deaths, the vast majority of the most severe cases are among older residents.

Further, even with the dramatic growth, only about 2 percent of fully vaccinated Minnesotans have tested positive. Overall, 14 percent of Minnesota’s 5.8 million residents have tested positive for the coronavirus since March 2020.


Dr. Andrew Badley, who leads the Mayo Clinic’s COVID task force, said the overall rate of breakthrough cases is in line with expectations of vaccine efficacy. No vaccine is 100 percent effective, he said.

Badley acknowledged growing evidence that protection from the vaccines decreases over time, especially in seniors and people with conditions that suppress their immune systems’ response to vaccines.

“Their immune response to the vaccine is not perfect. It’s also not zero,” Badley said.

“It is very rare to see someone who has been immunized and has a normal immune system to get sick enough to need to go into the hospital,” he added.

State health department data support that view. Ehresmann says that while the average age of a Minnesotan with a breakthrough case is 49, those who end up in the hospital are 74 on average, and those who die are 81 on average.

What the latest research suggests

A study on the midterm effectiveness of vaccines, published Nov. 4 in the journal Science , examined health care data from 780,000 veterans. It found the efficacy of all three vaccines dropped dramatically six months after people received their last dose — especially for the one-shot Johnson & Johnson.

After six months, the Moderna vaccine’s effectiveness had dropped from 89 percent effective to 58 percent; Pfizer’s from 87 percent to 43 percent; and the Johnson & Johnson from 86 percent to just 13 percent, researchers from the Public Health Institute, the University of Texas and Veterans Affairs found.

One of the study’s authors, Barbara Cohen, said their study adds to the mounting evidence that the pandemic is far from over. She hopes the new information will help people better protect themselves and their families against more dangerous variants.


“It is very clear that vaccines are protective,” said Cohen, a director and senior researcher at the Public Health Institute.

However, “the vaccines have declined in their ability to protect you fully from breakthrough infections. Our data shows that it looks like that’s because delta overwhelmed the vaccine,” she said.

Vaccine makers have not challenged these assertions. In fact, they’ve used similar evidence to strengthen their cases for providing booster shots to all adults.

A recent study of Israeli hospital data found booster doses refortified the protection vaccines initially provided — even against the delta variant.

Finally, while the vaccines and prior infection both protect against future disease, there’s growing evidence that vaccines give the immune system an added boost.

A recent report by the Centers for Disease Control and Prevention found patients who had previously tested positive for COVID-19 were 5.5 times more likely to catch the coronavirus again compared to those who were fully vaccinated.

Being vaccinated after recovering from a COVID infection provides even stronger protection, the report said.

What remains unclear

Despite the ongoing research into vaccine effectiveness and the impact of coronavirus variants, there’s a lot that remains unknown.

The delta variant is more contagious, but some uncertainty remains about whether it always causes more severe illness in both the vaccinated and the unvaccinated.

A lot of evidence points to yes.

Minnesota is more than three months into its fourth wave of coronavirus cases, and the current rate of infections, hospitalizations and deaths is the highest it’s been all year. Nearly all new infections are caused by delta.

Dr. Hannah Lichtsinn, who works at a Minneapolis clinic, said she’s regularly seeing fully vaccinated patients with breakthrough cases that “don’t just have the sniffles.”

Patients are complaining of fever, body aches and other serious symptoms typical of COVID-19 infection.

“My reaction when I see them is, thank god these people are vaccinated,” Lichtsinn said. “If they are this sick with the protection from the vaccine, I can’t imagine how sick they could be without it.”

There also are ongoing questions about how long the renewed immunity from a booster shot will last. There’s growing talk that COVID-19 will become endemic.

Badley, of the Mayo Clinic’s COVID task force, thinks regular boosters may be necessary. “It’s too soon to tell how often that’s going to be,” he said.

Finally, delta may be the variant getting all the attention now, but coronaviruses constantly are evolving. With well under half the global population fully vaccinated, it’s apparent new variants will emerge.

That’s why health officials will continue to push vaccine holdouts to get the shot, even as more and more people who already are inoculated get boosters.

“We need to continue to emphasize the importance of primary vaccination, because although there’s definitely evidence of waning immunity, people who have been vaccinated have some protection,” Ehresmann said. “People who have not been vaccinated have zero protection.”

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