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Measles cases in Pennsylvania topped 900<\/a> this week, as the state\u2019s largest outbreak in three decades<\/a> continues to spread from rural Lancaster County communities into Philadelphia\u2019s suburbs. <\/p>\n The Pennsylvania Department of Health reported 114 new cases in the last week, reaching a total of 903 cases this year as of Monday. That includes 83 cases in Chester County<\/a>, the only county in the Philadelphia region where the virus is actively spreading in an outbreak that is also the nation\u2019s largest currently<\/a>. <\/p>\n Chester County\u2019s outbreak<\/a> has emerged as the third largest in the state, having more than tripled in size since early August.<\/p>\n Physicians and local health officials say the current case count is likely an undercount, with many cases going unreported.<\/p>\n Cases in Chester County have largely been concentrated along its western border with Lancaster County. But health experts expect the virus to continue to spread statewide. More than half of Pennsylvania\u2019s counties already are affected, as vaccination rates have declined<\/a> in many communities.<\/p>\n Overall, Chester County has a 94.5% measles vaccination rate among kindergarten students, just below the 95% of people scientists say need to be vaccinated to prevent the virus from spreading within a community. <\/p>\n The so-called herd immunity<\/a> threshold for measles is high because the virus is so contagious: Nine out of 10 unvaccinated people who come into contact with measles will get sick.<\/p>\n In response to the outbreak, the county\u2019s health department<\/a> has ramped up communications and outreach, sent nurses into schools and close-knit communities, and held pop-up clinics. The overarching goal of all these efforts: Get more people vaccinated.<\/p>\n Two doses of the measles vaccine offer 97% protection against the virus. <\/p>\n \u201cWe are working all the time with organizations, businesses, community leaders \u2014 whoever we can get the attention of in the communities impacted,\u201d Jeanne Franklin, the county\u2019s health director, said in an interview last week. <\/p>\n The county has administered 538 measles, mumps, and rubella (MMR) vaccines this year, most of them since the outbreak began in April, she said. <\/p>\n Measles spreading in Pennsylvania<\/p>\n Across Pennsylvania, at least one measles case has been reported in 39 of 67 counties. <\/p>\n Almost all have been among unvaccinated individuals. The state reported four rare \u201cbreakthrough\u201d cases of measles<\/a> in vaccinated people earlier this month. <\/p>\n The virus\u2019 most common symptoms include a fever, cough, runny nose, watery eyes, and a telltale splotchy red rash. <\/p>\n Most people recover, but rare complications can be deadly.<\/p>\n