ST. PAUL — The number of Minnesotans hospitalized and newly infected with the coronavirus continues to increase as the more contagious delta variant spreads. State and federal health officials say COVID-19 vaccines remain the best defense against the deadly virus.

On Friday, Sept. 10, alone, the state reported 2,050 new infections and nearly 700 hospitalizations. There were also 18 more deaths.

“Vaccination against the delta variant still remains highly effective,” Dr. John O’Horo, a Mayo Clinic infectious diseases expert, said during a recent news conference. “Even though there are reports of breakthrough cases, they tend to be far less severe and far less frequent.”

O’Horo added that states with higher rates of vaccination have not been hit as hard by the delta strain as those with lower rates.

President Joe Biden recently announced vaccines will be mandatory for about 100 million residents who work for the federal government or larger employers. A Centers for Disease Control and Prevention study released Friday found unvaccinated people are 11 times more likely to die of a coronavirus infection than those who are vaccinated.

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Here’s the latest on vaccine safety, efficacy and breakthrough cases:

What is the rate of breakthrough infections?

It’s hard to tell for sure. In early summer, before the delta variant was as prevalent, Minnesota health officials said nearly all new cases were in people who were not fully vaccinated.

Delta is as much as 60 times more contagious than the original strain of SARS-CoV-2 that caused COVID-19. Now that delta is everywhere, there are more breakthrough cases being reported in fully vaccinated residents, but severe cases remain rare.

Officially, the state Department of Health reports 15,819 breakthrough cases out of 3 million people who are fully vaccinates, or 0.52 percent. Of those, 957 have been hospitalized and 93 have died.

Health officials say nearly all severe breakthrough cases are in seniors. The vast majority of the rest are likely asymptomatic people who got tested to travel, head back to school or for some other reason other than feeling very sick.

“It still seems to be mostly driven by milder disease,” O’Horo said. “Although it is becoming a little blurrier now, many of these cases are minimally symptomatic.”

Will booster shots be needed?

Vaccinations train the immune system to recognize invaders and combat them with antibodies. Over time, those protective forces can decline and the erosion is often faster in older people with less robust immune systems.

The coronavirus is also changing, as evidenced by the delta variant now driving infections as well as emerging variations. The most successful strains of the coronavirus are good at avoiding antibodies developed after vaccination or through prior infection.

To boost the body’s immunity, additional doses of vaccines may be needed. That’s already been OK’d for people who have compromised immune systems because of cancer or other diseases.

Later this month, federal officials are likely to release new guidance on booster shots for others. The World Health Organization has opposed boosters because much of the developing world has not had a chance to get even one dose of vaccine.

In the U.S., boosters will likely be focused, at least initially, on older people who have less robust immune systems.

O’Horo says there are two main hypotheses for why boosters might be needed:

“There’s some degree of immune waning. This is true of virtually any vaccine, that over time it won’t stay at its peak efficacy. As well as the rise of the delta variant.”

He added: “In all honesty, it is probably some combination of both.”

What's the latest on safety?

Three vaccines are currently approved in the U.S. The Moderna and Johnson & Johnson vaccines have emergency authorization and the Pfizer shot now has full approval from the Food and Drug Administration.

Coronavirus vaccines have been researched and vetted probably more than any other treatment in modern history. Doctors, medical researchers and federal officials have all concluded the shots are safe and highly effective.

Dr. Elyse Kharbanda, a senior investigator at HealthPartners Institute, says her institution is part of a broad network researching vaccine safety. So far, they have found no “concerning safety signals,” including during pregnancy.

Researchers from eight health networks recently published a review of health outcomes from more than 6 million people who received vaccines.

“I think overall, the vaccines have a reassuring safety profile,” Kharbanda said. “When outcomes of concern have arisen, they have been extensively studied. I think the vaccines have been more thoroughly studied than probably any other vaccine that is routinely administered.”

What about side effects?

While there have been thousands of reports of medical conditions and other adverse health events occurring after vaccination, health officials caution that does not mean those conditions were caused by the vaccine.

“There will always be anecdotes that people bring up of unfortunate events happening after vaccination, that doesn’t mean that was the cause of (a death) or that, overall, vaccination increases risk,” Kharbanda said.

She noted that the health network’s recent study found a slightly elevated number of cases of inflammation of the heart muscles and the sack around the heart in patients between the ages of 12 and 39. Some of those cases resulted in hospitalizations, and while there wasn’t a significant link between COVID-19 vaccinations and the condition, it continues to be studied.

Health officials have cautioned against using reports to the federal Vaccine Adverse Event Reporting System, or VAERS, as hard evidence of vaccine side effects.

The system, which is managed by the FDA and CDC, is designed to act as a catch-all for possible vaccine reactions that may not have been considered by doctors or medical regulators. Anyone can file a report with the system and many reports are unverified.

“The goal of VAERS is to be able to identify safety concerns that haven’t previously been considered,” Kharbanda said. “They accept reports from anybody. It is not meant to conduct definitive safety studies.”

Why do vaccinated people need to mask?

While vaccination continues to be the best way to avoid COVID-19 infection and severe illness, there is growing evidence that vaccinated people can still spread the coronavirus, especially the more virulent delta strain.

That’s why state and federal health officials continue to urge everyone — vaccinated or not — to practice virus mitigation measures such as masking in crowded places, social distancing, staying home when ill and getting tested if exposed to someone who’s sick.

Fully vaccinated people, two weeks after the last dose, do not need to quarantine after a possible exposure if they don’t have symptoms of illness, according to state and federal guidelines.

Health officials recommend that unvaccinated people quarantine for two weeks if they were in close contact with an infected person for 15 minutes or more.