California Pertussis Epidemic Reveals Vaccine Gap

Ricki Lewis, PhD

July 19, 2012

July 19, 2012 — A new study published online today in the Journal of Pediatrics examines the pertussis epidemic that occurred in California in 2010 and suggests strategies for protecting the most vulnerable individuals from this bacterial respiratory infection.

Kathleen Winter, MPH, from the California Department of Public Health (CDPH), Richmond, and colleagues analyzed 9154 cases reported in 2010, including 809 hospitalizations and 10 fatalities.

Of the hospitalized patients, 584 (72%) were younger than 6 months. The median age of onset among all hospitalized cases was 2.6 months. Nine of the fatalities were infants younger than 2 months, and the 10th case was a premature infant who had received the first vaccine and was 2 months old.

Nine of the 10 young infant fatalities were Hispanic. Past studies have noted overrepresentation of Hispanic infants among pertussis cases, but the causes for that disparity are unknown, according to Ms. Winter and colleagues. They suggest that an important factor may be larger household size, and therefore more contacts.

Among patients between the ages of 1 and 18 years, whites had the highest incidence. Overall incidence per 100,000 population was 26.6 for Hispanics, 21.0 for whites, and 10.1 for blacks.

The individuals most vulnerable to pertussis are babies too young to have received their first vaccinations, which include the diphtheria, tetanus, and acellular pertussis (DTaP) vaccination. Current recommendations include 4 doses of DTaP between the ages of 2 and 18 months, followed by a dose given between the ages of 4 and 6 years; a booster vaccine, called Tdap, is recommended for individuals 11 to 18 years, and for adults. Gaps in vaccination may fuel disease resurgence because pertussis is highly transmissible (just 1 case can infect from 12 to 17 others).

The researchers found that 9% of the 4415 cases in individuals aged 6 months to 18 years, for whom vaccination information was available, were not vaccinated, and 1621 (37%) had missed at least 1 dose. Other cases occurred in preadolescents in whom booster strength may have been waning because of the replacement of whole cell vaccine with the less efficacious acellular vaccine in the 1990s.

In the wake of the 2010 epidemic, CDPH developed and disseminated press releases, clinical guidance updates, and educational materials to the public about appropriate vaccination to protect the youngest infants. The US Centers for Disease Control and Prevention also recommended Tdap immunization for all unvaccinated pregnant women past the 20th week of pregnancy.

The state provided free vaccines for postpartum women and people in direct contact with young infants. Unpublished data from CDPH indicate that before free vaccine distribution, only about a quarter of California birth hospitals offered Tdap to postpartum women.

Incidence of pertussis in California in 2010 was the highest in more than 60 years. The researchers recommend analysis of measures implemented to fill the vaccine gaps, identified as postpartum women, young infants, and preadolescents in whom boosters may have lost efficacy.

The researchers conclude, "[W]ithout a vaccine that is more efficacious and confers lifelong immunity, pertussis will continue to be a threat to the lives of young infants." Until such vaccines are developed, they add, other strategies, such as vaccination of close contacts, should be evaluated.

The authors have disclosed no relevant financial relationships.

J Pediatrics. Published online July 19, 2012.


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