Montana's Missoula County has recorded 169 pertussis cases in 2019, and the county health department issued a warning Sept. 5 that the number of cases is growing as children return to school, according to The Missoulian.
The number of confirmed cases is a record for the county, where the past record was 23 cases in 2012. There were 131 cases as of May 2019, and there have been about two new cases per week since then. Health officials expect an uptick in cases as the school year begins.
The pertussis vaccine requires 80 percent herd immunity to be effective, but Missoula is not even close to that percentage, said Cindy Farr, the county's health promotion director. The vaccine is also only about 70-80 percent effective over the first year and becomes less effective as time passes.
The outbreak will not be considered over until 42 days go by with no new cases.