The disease, caused by the bacterium Bordetella pertussis, is spread via coughing or sneezing and is highly contagious. On average, one infected person can spread the disease to 12 to 17 others, says Lynnette Mazur at the University of Texas Medical School at Houston.
Eight people have died of it in California so far this year, and some 3000 cases have been reported – seven times the number over the same period last year. Idaho, Texas, South Carolina and Michigan have also reported increases.
James Cherry at the University of California, Los Angeles, Medical Center says this looks set to be a particularly large outbreak, but it is not totally unexpected. Outbreaks occur every two to five years because the vaccine given to children is only 80 per cent effective and only protects up to about age 10, he says. What's more, few adults receive boosters.
The greatest danger is to unvaccinated infants: seven of the eight deaths in California were babies under two months of age.
Some areas of California are vaccinating all family members who might be in routine contact with a baby in its first months – a strategy called "cocooning". But all adults could do their part to minimise the outbreak, says Mazur. "Take a look at your immunisation record," she says.
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