Whooping cough in California reaches epidemic proportions

A pertussis (whooping cough) epidemic has been declared in California, according to the California Department of Public Health (CDPH).

As of June 10, there have been 3,458 cases of pertussis reported to CDPH in 2014, more than were reported in all of 2013. More than 800 new cases have been reported in the past 2 weeks. Pertussis is cyclical and peaks every 3 to 5 years. The last peak in California occurred in 2010, so it is likely another peak is under way.

The Centers for Disease Control and Prevention (CDC) defines an epidemic as the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time, according to Gil Chavez, MD, MPH, State Epidemiologist and CDPH's Deputy Director of the Center for Infectious Diseases.

In recent non-peak years, an average of 80 to 110 cases of pertussis occur each month in California so this is considered the baseline level of disease activity. In both April and May 2014, more than 900 cases have occurred in each month which is more than 500% over the baseline.

A number of US states that did not have pertussis epidemics in 2010 had epidemics in 2012, according to Dr Chavez. In 2012, there were more cases nationally than had occurred in 50 years, including 18 infant deaths.

“Pertussis vaccine is the only tool we have to prevent pertussis,” he said. “Although the current acellular vaccines do not provide protection for as long as the whole-cell vaccines that were used from the 1940s until the 1990s, they still prevent many cases of pertussis and there would be many more cases of pertussis if persons were not vaccinated.”

The incidence of pertussis in the pre-vaccine era was approximately 150 cases per 100,000 population, 6 times greater than the incidence in the 2010 California pertussis epidemic (25 per 100,000 population). The current incidence in California is 9 per 100,000 population.

“There is preliminary evidence that young infants whose mothers are vaccinated against pertussis during pregnancy are less likely to become ill with pertussis,” Dr Chavez said. “The vaccination of pregnant women is the most important method to protect young infants from pertussis infection before they are old enough to be immunized. Tdap vaccination is still the best tool we have available to prevent individual cases of pertussis among adolescents and adults.”