In Kentucky, infection has prompted at least 385 schools across 44 districts to cancel in-person classes for a day or more, accounting for three-quarters of all such disruptions across the country this academic year, according to a recent from the school data service Burbio. Tennessee was the state with the second-most closures, with just 46 schools across six districts affected.

Why the Bluegrass state stands out as such an outlier remains something of a mystery to officials. 

鈥淚 really cannot imagine why Kentucky is experiencing more closures than other states,鈥� said Ballard County schools Superintendent Casey Allen, whose district for two days in early November when influenza rates were high.

Unlike during the peak of the pandemic, disruptions this year have tended to last only a few days and have mostly been spurred by the flu, not COVID. Flu spread in Kentucky has been some of the highest in the nation, according to the Centers for Disease Control and Prevention. Another 10 states are also at the highest level of risk.

In Ballard County in early November, student and staff attendance fell to around 70%, Allen explained in an email. With such large swathes out sick, he felt his hands were tied.

鈥淟ike many of my colleagues, I consider closing schools due to illness to be a last-ditch effort to stop the spread of an illness,鈥� he said. 鈥淥ur district values in-person classes and works hard to have our students in school with a teacher in front of them.鈥�

With so many instructors unable to work, the district did not pivot to virtual learning and will have to make up the days at the end of the year. Powell County schools shut down for a full week and will also have to make up the days, said Superintendent Sarah Wasson. Several other districts used virtual instruction while classrooms were shuttered.

鈥淔or some reason, flu season seems to be earlier this year than in previous years. This particular flu [strain] appears to have a longer impact period than previous years,鈥� said Jamie Weddington, superintendent of Lewis County Schools, which also briefly closed.

Districts can receive approval for up to 10 days of remote learning per year from the state education commissioner, explained Toni Tatman, a Kentucky Department of Education spokesperson.

She questioned the accuracy of the Burbio figures and declined to comment on why her state鈥檚 rate of school closures this year has been higher than other states with similar levels of flu infections.

鈥淲e have no reason to believe Kentucky’s numbers would be any different than other states,鈥� Tatman said.

Burbio collects its data through weekly audits of news stories and communications from a list of 5,000 school districts that account for 70% of the nation鈥檚 students, the company鈥檚 co-founder Dennis Roche explained. Though Burbio鈥檚 data does not capture every closure, he said his team鈥檚 10 researchers could not have systematically over-represented disruptions in Kentucky compared to those in other states.

鈥淲e鈥檝e now been doing this 鈥� for over two years,鈥� he said. 鈥淲e really go pretty deep on this stuff.鈥�

The Kentucky School Boards Association has also tracked illness-related school closures in the state, documenting 66 districts affected, an even higher tally than Burbio鈥檚. Spokesperson Josh Shoulta said he couldn鈥檛 鈥渟peculate鈥� on why Kentucky鈥檚 numbers have been higher than other regions because the organization hasn鈥檛 tracked closures outside its own state.

The Kentucky closure numbers arrive during what observers have called the first 鈥溾�� school year since COVID initially hit in March 2020, with days- or weeks-long school shutdowns now largely a thing of the past. Still, in another reminder of the pandemic鈥檚 long shadow, officials leading Sacramento, California鈥檚 school system said they could soon reinstate a requirement that students and staff should COVID rates continue to increase in the city.

On Dec. 8, New York state health officials that kids should wear masks in schools, explaining in a letter that, “A host of respiratory viruses, including influenza, RSV and COVID-19, have taken hold in our state and in most of our communities. These viruses, while often manageable, can cause serious outcomes, especially for children.”

Earlier this month, the virus was detected in the United States for the first time since 2013. for poliomyelitis, the scientific name for the illness; where he contracted it is still unclear. 

Although the last case that originated in the U.S. occurred in , unvaccinated people are at risk. and missed doctor鈥檚 appointments because of COVID-19 mean some kids aren鈥檛 getting the protection . 

The Centers for Disease Control and Prevention children receive four doses of the polio vaccine between 2 months and 4 years old, and schools and child care facilities in every state require the shots, which are safe and effective, before children can attend. But around the United States, vaccination rates vary widely. 

Every state allows children to be exempted from the immunizations for medical reasons. Washington, D.C., and 44 states allow religious exemptions, and 15 allow philosophical exemptions, according to the . 

Within a state, vaccination coverage can vary widely as well. In Rockland County, New York, for example, where the first case of 2022 was detected, about , according to recent state data. In Nearby New York City, where the virus has , the overall rate is about 86%. Even within the city, rates differ, with some neighborhoods reaching higher than 99% while others are .

The map below shows the in each state who received the recommended series of vaccines 鈥� which includes immunization against polio as well as measles, mumps, rubella, varicella and other illnesses 鈥� by 35 months old.

Share of Children Who Have Received Recommended Series of Vaccines by 35 Months

For herd immunity from polio, about 80% of people in a community need to be immunized, according to the . The vaccination rate associated with herd immunity 鈥� the minimum share of people who must be protected to stop an illness from spreading鈥� varies by disease. 

Polio is often asymptomatic but . In rare cases, the virus can result in muscle weakness, paralysis or death. Anyone who is not vaccinated can contract polio, but it most commonly affects children under 5 years old, .

This map shows the percentage of kindergartners in each state who have received the recommended doses of the polio vaccine.

Share of Kindergarteners Vaccinated Against Polio By State

In the early 1950s, before the vaccine was available, polio caused 鈥� of paralysis each year,鈥� according to the CDC. Schools played a vital role in vaccine trials in the later part of that decade.

One reason vaccines are less popular than they were when polio was widespread is because they work, David Oshinsky, a medical historian at New York University and the author of Polio: An American Story, .

Effective vaccines have “obliterated evidence of what [polio] can cause: kids on crutches, in wheelchairs, in iron lungs,” he said. “I remember seeing the occasional empty desk in school because a child had died. People had seen polio every summer, and they wanted kids vaccinated as soon as possible.”

But on Aug. 30, the 2,800-student district began requiring students to wear masks outside as well as in the building 鈥� and hasn鈥檛 had to send a whole classroom home since. The new policy was prompted by the state鈥檚 revised for unvaccinated students, which allow asymptomatic students to stay in school if they meet several conditions, including wearing masks both inside and outside.

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鈥淲e are optimistic it is working,鈥� said Kristie Towne, manager of board and superintendent operations for the Solana district, part of San Diego County. 鈥淭丑别 policy is meant to keep as many children in school [and] in class as much as possible.鈥�

With the recent rise in positive cases due to the more transmissible Delta variant, districts like Solana Beach are now enforcing additional measures 鈥� policies that go beyond recommendations from most state health departments and the which says masks aren鈥檛 needed during recess. The Los Angeles Unified School District was among the first to institute the practice and several other California districts have followed suit. Others as far as Vermont and North Carolina have instituted similar measures but are targeting them to younger students or athletes. One problem: The research behind such moves is pretty thin.

鈥淥utside, there鈥檚 an infinite volume of air to dilute the virus,鈥� said Dr. Dean Blumberg, a pediatric infectious disease expert at the University of California, Davis.

Dr. Benjamin Linas, a Boston University epidemiologist, warns that outdoor masking could even be counterproductive.

鈥淚f there is any hope of successfully implementing masks when we need them 鈥� indoors during Delta surge 鈥� then we cannot insist on masks when we do not need them, and we should not routinely 鈥榬ound up鈥� when not certain,鈥� he said. 鈥淭oo much masking is real.鈥�

Advocacy groups that were already fighting the state鈥檚 mandate that students wear masks indoors argue that requiring them outdoors further hinders children鈥檚 social development.

鈥淥utdoors our kids need to be breathing fresh air. They need to have social interaction and share smiles,鈥� said Sharon McKeeman, who founded and in July filed , with Reopen California Schools, against California Gov. Gavin Newsom, the state health department and other officials. 鈥淭丑别se restrictions are arbitrary, and they are infringing our kids鈥� rights.鈥�

The measures came as some districts faced criticism for quarantining too many students without symptoms.

In August, in Los Angeles and other districts missed class and did not always have access to remote learning. Other California districts requiring masks outside include the 12,000-student Palo Alto Unified School District, where the most shows two cases districtwide, and the 9,600-student , which had 27 cases in August and seven so far in September.

鈥楾he benefits are uncertain鈥�

Some opponents of mask requirements note that the , which President Joe Biden as soon as he became president, doesn鈥檛 recommend masks at all for children 5 and under.

A growing body of research on transmission of the virus shows that the proportion of cases originating outside are well below 10 percent and could be even less than 1 percent, according to a in the Journal of Infectious Diseases.

International studies provide further evidence of significantly low risk. A found that out of 7,300 cases, one outbreak resulting in two cases was linked to an outdoor conversation between two people. An showed that about one in 1,000 cases was due to outdoor transmission.

Most outdoor cases are linked to lengthy interactions between people or crowded events, studies show.

鈥淚 am having a very hard time thinking of when a school would generate such an opportunity for transmission,鈥� Linas said. 鈥淚t is not recess or outdoor classwork. Perhaps if a school had an outdoor pep rally in a relatively small stadium with full bleachers and kids on the field, too. I am struggling to come up with a realistic scenario.鈥�

Experts stress that with the Delta variant, local vaccination rates of those 12 and above should guide decisions about whether additional caution is needed.

That鈥檚 why Andrew Hayes, a school board member in the Lakeside Union Elementary School District in San Diego County, questions the governor鈥檚 inside mask mandate to begin with.

鈥淭丑别 about being at an 80 percent vaccination rate, but we are still having all these mitigation strategies everywhere,鈥� Hayes said. 鈥淚 understand that people want to follow the experts, but they aren鈥檛 allowing the experts in education to make decisions.鈥�

His district has not yet required masks outside, but surrounding districts have.

Hayes is among in California who want to loosen local mask requirements in violation of the statewide mandate requiring students to wear them indoors. The California Department of Public Health on Aug. 23 sent districts stating they could face fines and civil lawsuits if they don鈥檛 enforce masking.

The dynamic is the opposite of that in Florida, where districts mandating masks are locked in a protracted legal battle with a Republican governor who says parents should choose.

California isn鈥檛 the only state where some districts are going above and beyond CDC guidelines, which state: 鈥淚n general, people do not need to wear masks when outdoors鈥� for play, recess and physical education. But other examples are more targeted.

The , near Burlington, Vermont, requires masks outdoors for students in K-5 if they can鈥檛 socially distance. The district is requiring masks outside for elementary and middle school students, and the district in North Carolina requires athletes to wear masks outside when they鈥檙e not actively participating in a game or practice.

In California, McKeeman, with Let Them Breathe, said even in districts that don鈥檛 require students to wear masks outside, 鈥渢here鈥檚 still a lot of enforcement to keep it on anyway.鈥�

Some experts recognize the challenges teachers and other school staff members face when children are constantly taking masks on and off. Blumberg, who said he still wears a mask when he goes to the farmer鈥檚 market, noted that many classroom buildings in the state鈥檚 schools are connected by outside hallways.

For the sake of consistency, he said, 鈥淚t鈥檚 easier to just say, 鈥楳ask while at school and don鈥檛 think about it.鈥欌��