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Minnesota is almost halfway to vaccinating 80% of its eligible population against COVID-19, but whether it can reach that goal — or even needs to get that far to stifle the pandemic — is unclear.

State leaders said the 80% COVID-19 vaccination goal is an estimate, based on limited knowledge of the new SARS-CoV-2 coronavirus and more experience with other pathogens. Vaccination rates of 80% and 95% were needed, respectively, to tamp down polio and measles outbreaks.

"It was a rough estimate to get to a point from which we could then evaluate progress in reducing spread," said Kris Ehresmann, state infectious disease director. "It was based on what we know about other diseases and what we thought we knew about SARS-CoV-2."

Nearly 2.3 million people, or 52% of eligible Minnesotans 16 and older, have received vaccine and more than 1.6 million have completed the one- or two-dose series. The latter number equates to 37% of the eligible population.

Health officials worry that the next phase could be the toughest, though, as vaccination moves beyond the highly motivated recipients who are at elevated infection risks due to their occupation, or are more likely to suffer severe COVID-19 due to their age or health conditions.

Leaders at Mayo Clinic and other health care institutions nationwide have noticed weakening in the vaccine uptake rate. Appointments are still getting filled, but they aren't snatched up as quickly. Mayo in conjunction with Cleveland Clinic and 60 other medical providers launched a promotional campaign Tuesday to get vaccine-hesitant people to seek shots.

Waiting for others with greater need to go first is no longer necessary, said Dr. Melanie Swift, co-chair of Mayo's COVID-19 vaccination program. "Vaccine is not at this point a personal individual ethics test. It is really time to get that vaccine if it's available to you. Get the first vaccine you can get. You are helping everybody in your community by doing that."

Mayo modeling showed that Minnesota's current vaccination rate would not be enough, given the spread of more infectious viral variants, to prevent another COVID-19 wave that could be as widespread as last winter's surge. If Minnesota were already at 75%, the modeling suggests, the current wave would dissipate.

Even a 75% goal leaves a narrow margin when considering the latest national polling data showing a hesitancy rate of around 23%, Swift said. While hesitancy has declined, some health officials fear the nationwide pause on the use of the Johnson & Johnson vaccine to review extremely rare blood clots in six recipients could turn sideline skeptics into die-hard opponents.

"I don't know if we'll be able to convince those folks," Swift said, "but if we can convince everyone else, then we could get to about 75%, which would make things better for the rest of us."

The goals are based on the concept of herd immunity — that vaccination or natural immunity in enough people will give the virus too few opportunities to spread.

The best local evidence of this effect comes from long-term care facilities, which were prioritized for limited, early supplies of vaccine. Final doses had been administered as of April 8 to 82% of Minnesota's 19,042 skilled nursing home residents and 87% of 34,179 assisted-living residents.

Weekly infections among these residents dropped from more than 1,400 in mid-December when vaccine became available to 28 in the first full week of April.

Some health officials expect herd immunity to come before an 80% vaccination rate — noting that COVID-19 activity dropped substantially in Israel once it reached 50%.

One crude calculation of the vaccination rate needed to achieve herd immunity is based on the R-nought statistic, which estimates how many people a carrier would spread the virus to.

At recently published rates of 2.5 for SARS-CoV-2 in general population settings, a vaccination rate of around 66% would be needed, University of Minnesota epidemiologist Ryan Demmer said.

That leaves him encouraged for Minnesota, given the number of people who have been vaccinated and the rate of people with prior infections from the last pandemic wave who will have at least short-term immunity for a few months.

"After we're over 50 percent, we're really going to start to see some changes," he predicted. "By the time we get up to 60 to 65 percent, my anticipation is that the Minnesota numbers will really start to decline."

Herd immunity estimates of around 60 to 70% have been suggested by leaders of the World Health Organization and the U.S. Centers for Disease Control and Prevention, and yet it is wise to have more ambitious vaccination targets, said Gypsyamber D'Souza, an epidemiology professor at Johns Hopkins University in Baltimore.

Reaching 70% "doesn't mean infection will completely stop," she said. "Really bringing rates down to a low level likely [will] require higher vaccination rates, so it is appropriate that our goals for vaccination should be higher."

D'Souza stressed that herd immunity isn't a fixed concept and that the percentage of protection necessary varies depending on changes in the infectiousness of the virus or in public compliance with mitigation strategies such as social distancing.

"There is no one number/threshold for herd immunity," she said in an e-mail.

Minnesota is in a third pandemic wave fueled by more infectious variants of SARS-CoV-2. The state on Monday reported six more COVID-19 deaths and 1,292 more infections — bringing Minnesota's totals to 7,026 deaths and 557,665 known infections.

Minnesota hospitals reported 682 COVID-19 cases filling inpatient beds on Sunday, including 172 patients requiring intensive care. The surge in hospitalizations since early March has not been followed by a comparable increase in COVID-19 deaths, leaving health officials hopeful that vaccination of highest-risk groups is working.

Demmer said one problem in Minnesota's herd immunity goal is that COVID-19 vaccine hasn't been federally approved for anyone 15 or younger — a major gap when the virus appears to be spreading rapidly among teenagers and young adults.

"We won't get to herd immunity without kids getting vaccinated," he said.

Demmer said he hopes Minnesota will continue at its current rate of vaccination, and that approval of vaccine for children will happen by midsummer so they can be fully immunized before the start of the next school year.

Jeremy Olson • 612-673-7744