Pennsylvania has reported more than 1,000 measles cases and five deaths as the outbreak accelerates, renewing questions about whether residents are protected against the highly contagious disease. Patsy Stinchfield, executive director of the Measles Collaborative, said Pennsylvania had 12 cases in January and has recently seen more than 100 cases a week. She warned that similar spread could occur in other communities with low vaccination rates. New York state has declared a measles emergency, and more than 4,000 cases have been reported nationally this year — the highest annual total in 35 years, according to the figures cited by infectious disease experts. Two doses of the measles, mumps and rubella vaccine, known as MMR, are about 97% effective. One dose is about 93% effective. People who cannot confirm how many doses they received can check with a health care provider about their options. Guidance depends in part on age and exposure risk. People born before 1957 are generally presumed immune because measles was widespread before vaccination. Those born after 1957 who received a measles vaccine before 1968 may need another dose: some people received an older vaccine made with inactivated virus that was less effective. The Centers for Disease Control and Prevention advises people uncertain about that vaccine to discuss one or two MMR doses with a doctor. The standard recommendation for children changed in 1989 from one MMR dose to two, after officials found a single dose was not enough to prevent outbreaks. Two doses are also recommended for people at higher risk of exposure, including health care workers, college students, international travelers and people in close contact with someone who is immunocompromised. Adults in areas with substantial measles activity may want to ask a clinician whether a second dose is appropriate, said Dr. Larry Kociolek, a pediatric infectious disease specialist at Lurie Children’s Hospital of Chicago. Unvaccinated adults make up a significant share of people sickened this year, and people over 20 without immunity face a higher risk of serious complications. For people without vaccination records, a blood test can check for measles antibodies, but the test can sometimes show a negative result even when someone is protected, said William Moss, an epidemiology professor at Johns Hopkins University. Coverage for testing may vary. Most private insurance plans and Medicaid are required to cover another MMR dose at no cost, according to the CDC. Moss said getting an additional dose is generally a simpler option when a person cannot establish their vaccination history; people should discuss their circumstances with a health care provider. Two-dose vaccination also helps reduce the chance of exposing people who cannot receive the MMR vaccine, including pregnant people, severely immunocompromised people and infants younger than 6 months. Measles was responsible for 400 to 500 U.S. deaths a year before vaccines became available, Kociolek said.