The worst local outbreak of measles in decades, which likely will total at least seven cases, began when a toddler contracted the virus and began a chain of contagion among other unvaccinated children.

So far, four Monroe County children have been formally diagnosed with measles, a viral disease with symptoms that range from mild to life-threatening. None of the children had been immunized against the virus. A fifth child, also not vaccinated, is known to have had the measles but did not undergo confirmatory lab tests.

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Two more unvaccinated children have been exposed and presumably have measles or will show symptoms soon, said Ryan Horey, a Monroe County Department of Public Health spokesperson.

Seven cases is an exceedingly high number. Since 1995, only one other case of measles has been reported in a resident of Monroe County. That case involved a University of Rochester student who contracted the illness overseas in 2014.

Upstate New York in total has averaged just three cases of measles annually since 1995. With New York City included, the state's annual average has been about 15 cases.

Near-universal vaccination of American children made measles a rarity in this country by the late 1990s. Public health officials are concerned, however, that a growing number of parents who withhold the vaccine from their children for personal or religious reasons is bringing about a resurgence in the disease.

One of the nation's worst outbreaks in years is now underway in the lower Hudson Valley and New York City, where more than 200 cases have been diagnosed since last fall. A smaller clustering of cases has been reported in the Pacific Northwest.

The mini-cluster in the Rochester area is not related to either of those outbreaks. Instead, it began when the local toddler's family visited the eastern European nation of Ukraine, where vaccination rates are relatively low and measles is rampant.

The toddler was officially diagnosed with measles in early January.

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The child's parents had chosen not to have their child receive the first dose of measles-mumps-rubella vaccine, which is recommended when a child turns one, Horey said. A second dose is recommended between ages 4 and 6.

The first dose is believed to be about 93 percent effective in preventing measles, Horey said. The second dose boosts that to 97 percent.

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Prior to the time that the toddler was diagnosed with the illness, members of a family whose five children also are not vaccinated visited the toddler's home, Horey said.

One of the five children contracted the virus from the toddler. The child came down with measles but did not get a lab test.

The child passed the virus to two of his or her siblings, who got lab tests and were formally diagnosed with measles last week. The siblings attend Canal View Elementary School in Ogden.

That family's two other children, who are preschoolers, must have been exposed to the virus and very likely will get sick if they aren't already, Horey said.

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Finally, a child who lived next door to that family was reported Thursday to have measles. Like the others who are sick, that child also had not been immunized.

"All these kids were old enough to have had their first doses. None of them did," Horey said.

Data on vaccination rates as reported by schools indicate that a very high percentage of young people in the Rochester area have been immunized against measles and other communicable diseases.

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The most recently reported immunization rate at Canal View elementary, for instance, was 98.5 percent.

Horey was unable to say if there has been any recent increase in the proportion of local parents who are withholding vaccines from their children. He also was not certain of the reason why the parents of the seven infected children chose not to have their kids immunized against measles.

The state Department of Health, which is helping monitor for any further local cases, has seen more and more instances of measles being imported from other countries where the illness is endemic, said Jill Montag, a spokeswoman for the department. An ongoing outbreak among Orthodox Jews in Brooklyn apparently began after a child visiting Israel brought back the virus from that country.

Horey said health officials cannot predict with certainty whether anyone else has or will contract the virus from the local children who already have it.

The five children in the one family, including the two preschoolers who may not yet be symptomatic, have been voluntarily quarantined at home to reduce the chances they'll expose anyone else. A handful of other unvaccinated students at Canal View elementary were asked to stay home until the risk of infection had passed, Montag noted.

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Two of the children in the one family attended school during the time they may have been contagious, but there have been no reports of any further cases, Horey said.

SORR@Gannett.com