Whooping cough outbreak hits several US states

Health authorities in California are urging all citizens to immunise themselves against whooping cough amid what is set to become the largest outbreak of the disease in 50 years.

The state has seen a total of 2774 confirmed cases so far this year—seven times the number of cases last year—and eight babies have died. Seven of the babies were less than 2 months old and had not yet been vaccinated. The eighth baby had been born at 28 weeks and had received its first dose of the vaccine. A higher than normal incidence of whooping cough has also been reported in Michigan, New York, Pennsylvania, and South Carolina.

The incidence of whooping cough last peaked in the United States in 2004, when the rate was nine cases per 100 000 population. It then fell back to about half that in subsequent years; in 2008, the last year of complete data, the number of cases was 13 278, the Centers for Disease Control and Prevention (CDC) said.

The peak season for the infection is about to start, abetted by the return to school and close contact among children that facilitates spread of the bacterium. “Pertussis is so contagious that the average number of secondary infection cases that are produced by a single index case is 12 to 17,” said Lynette Mazur, a professor of paediatrics at the University of Texas Medical School, Houston.

Children who have been vaccinated and adults whose immunity from vaccination has waned can serve as carriers of the disease, even though they may exhibit mild or no symptoms.

California’s Department of Public Health is urging all Californians to ensure they are vaccinated against the disease, particularly if they are in contact with young children.

The roughly five year cyclical wave of infections is being made worse by a growing antivaccine movement that has led many parents to skip the standard childhood vaccinations.

“We are seeing some of this in schools with very low immunisation rates, where parents have decided not to vaccinate their kids,” said Mitchell Katz, director of health for San Francisco.

“The [San Francisco] Bay area is one of the highest with parent opt-out from vaccination. It is based on concern about autism among a group of very well educated parents who feel that these vaccines are not safe,” he said, even though there is no scientific evidence of such a link.

Infants with immature immune systems are most at risk of death from the infection. But elderly people and people in poor health are also at risk of infection, because their protective antibodies are likely to have waned. However, Medicare, the federal health insurance programme for people aged over 65, doesn’t cover or reimburse for vaccination of adults against whooping cough because the clinical trials were conducted only in children and the FDA’s label indication is only for children, though off-label use in adults is widespread.

Dr Katz said that San Francisco is providing “vaccine at a subsidised rate, specifically for that reason.” The city clinics are “focusing upon trying to increase vaccination rates, especially for people who come into contact with young children.”

The current vaccine has fewer side effects than earlier versions. But Dr Katz acknowledged that “the vaccine is not as good as some of the others” in terms of generating a durable immune response and is “more likely to wane over time.”