The number of cases of pertussis, or whooping cough in California this year is approximately five-times the number of cases in 2013, according to the latest data released from the California Department of Public Health (CDPH).

Pertussis incidence in California/CDPH
Pertussis incidence in California/CDPH

As of Nov. 26, California has reported 9,935 pertussis cases compared to for a state rate of 26.0 cases per 100,000 population. This compares to the 2013 numbers of 2,011 total cases and a state rate of 5.3 cases per 100,000 population.

This prompted the CDPH to declare a pertussis epidemic this past June. The last pertussis epidemic in California occurred in 2010, when approximately 9,000 cases were reported, including 808 hospitalizations and 10 infant deaths, for a statewide incidence of 24.6 cases per 100,000 population.

In a CDC MMWR analysis published Friday, disease incidence in California among infants aged <12 months was 174.6 cases per 100,000 during January 1–November 26, 2014, and was significantly higher among Hispanic infants and lower among Asian/Pacific Islander infants than among white, non-Hispanic infants.

Of 6,790 cases with available data, 347 patients had been hospitalized, including 275 (79%) who were aged <12 months, of whom 214 (62% of those hospitalized) were aged <4 months. Among hospitalized infants aged <12 months with complete information, 33% required intensive care; few (24%) had received any doses of diphtheria, tetanus, and acellular pertussis vaccine (DTaP). One death was reported in an infant aged 5 weeks at the time of illness onset. Two additional fatal cases in infants who became ill in 2013 were also reported in early 2014; both were aged <5 weeks at the time of illness onset, and one was hospitalized for more than a year before succumbing to pertussis-related complications.

Researchers continue to point to the acellular pertussis vaccine and the waning immunity that is provided.  They write that data available since the 2010 epidemic indicate that immunity conferred by acellular vaccines, particularly when used for the primary series, wanes more rapidly than that conferred by older, whole-cell vaccines that were used in the United States from the 1940s to the 1990s.

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Because of vaccine safety concerns related to whole-cell pertussis vaccines, acellular pertussis vaccines were developed and recommended in 1992 for the 4th and 5th doses of the pertussis vaccine series and for all 5 doses in 1997. Acellular pertussis vaccines are less reactogenic than whole-cell vaccines, but the immunity conferred by them wanes more quickly.

As long as currently available acellular pertussis vaccines are in use, it is likely that the “new normal” will be higher disease incidence throughout pertussis cycles. The number of reported cases in 2014 has surpassed that of the 2010 epidemic and represents the most cases reported in California in nearly 70 years.

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