I N 2000 AMERICA declared measles “eliminated”, meaning that the virus was no longer indigenous and any new infections were linked to strains brought in from abroad. In the following decade measles in America remained rare. Now cases are on the rise again. There were 372 in 2018, the second highest number since 1996. Over 200 were reported in the first two months of this year. Though the disease is rarely deadly, it often requires hospitalisation.

Most recent large outbreaks have been in insular religious or immigrant groups, such as the Amish, Orthodox Jews and Somali-Americans. Some had been lectured or leafleted by crackpots who claim that vaccines are harmful. They are easy prey for such conspiracy theories because language and cultural barriers keep them at a distance from mainstream health care. Low vaccination rates have made them hotspots for outbreaks, often ignited by measles picked up on visits to relatives in countries where the disease is widespread.

Imported cases have arrived from more than 75 countries, sparking outbreaks across America. Rapid action by public-health SWAT -like teams keeps the virus from spreading. The teams trace everyone who has been near the measles patient in the eight-day contagious period—and make sure that each contact is quarantined or immunised. Nine in ten people who are not immune would contract measles if exposed to it. The virus can linger in the air for hours.

Containing outbreaks is becoming harder. The number of measles cases contracted in America for each imported case is increasing. A tally in 43 states in 2014-15 found that in nearly half of counties the rate of measles vaccination of children entering kindergarten was below the 95% needed to prevent an outbreak. Things may have got worse since. Almost all states allow parents to exempt their children from jabs by declaring a religious objection to vaccines; 17 states allow “philosophical” objections, too. In 2017-18 such non-medical exemptions were used for 2.2% of schoolchildren, double the rate in 2010-11.

As long as parents’ choice is put before public health, stopping measles from spreading in America will be a laborious, costly task. Washington, one of the states battling an outbreak now, has spent more than $1m to curb contagion since an imported measles case arrived in January.

Sources: CDC; “County-level assessment of United States kindergarten vaccination rates for MMR” by S.A. Kluberg et al. 2017;“International Importations of Measles Virus into the United States During the Postelimination Era” by Adrian Lee et al. 2018; The Economist