(NEW YORK) — Nearly all the patients affected by an ongoing chickenpox outbreak in New York City are unvaccinated, according to a report published Thursday.

The outbreak was first identified by city health officials in October 2022 and was found to be among people who recently migrated from or through central and South America and lived in a shelter or residential facility.

The report, co-authored by several New York City agencies and the Centers for Disease Control and Prevention, looked at patient interviews and reviews of medical records conducted by the city’s Department of Health and Mental Hygiene (DOHMH).

Between September 2022 and March 2024, 873 people who tested positive for chickenpox were linked to the outbreak, according to the report.

Of the patients, 91.9% had no documentation of receiving a chickenpox vaccine prior to the beginning of symptoms. Data showed 6.8% received at least one dose and just 1.4% had received two doses.

There are two chickenpox vaccines licensed in the United States. The first contains only the chickenpox vaccine and can be given to children in two doses: the first at 12 to 15 months old and the second at 4 to 6 years old.

The second contains a combination of the chickenpox vaccine and the measles, mumps and rubella vaccine and is authorized for children between 12 months and 12 years old.

Studies have shown the vaccine that protects just against chickenpox is 82% effective against any form after one dose and 92% effective after two doses, according to the CDC. Receiving one or two doses is 100% effective against severe chickenpox.

The chickenpox vaccination program in the U.S. began in 1995 and was a major success, reducing overall annual cases by 97%, the CDC said.

The report also provided breakdowns by age and sources of transmission. The majority of cases — 53% — were among people between ages 4 and 18. Adults above age 18 made up 29.4% of cases and children under age 4 made up 17.5% of cases.

Among the 780 patients who knew where they had contracted the disease, 41.3% were exposed at shelters or residential facilities while 39.4% of cases were imported, meaning the incubation period occurred before arriving in New York City. School transmission accounted for just 1.2% of cases.

According to the report, many recent migrants come from countries that don’t have routine chickenpox vaccination programs or have high incidence of chickenpox. Of the countries that do include chickenpox vaccination as part of routine immunizations, programs may have been disrupted due to COVID-19 or political instability.

“This outbreak highlights the importance of limiting transmission by achieving and sustaining high [chickenpox] vaccination coverage and the need for rapid, large-scale vaccination efforts in light of ongoing importations and exposures in U.S. shelters and residential facilities,” the report’s authors wrote.

Some people may experience complications from chickenpox and require hospitalization, but hospitalizations and deaths from chickenpox are rare in the present day due to the U.S. vaccination program, according to the CDC.

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