The pertussis epidemic continues in California, which has seen 5,658 confirmed, probable, and suspected cases as of this week.
That is the most cases seen in the state since 1950, when there were 6,613, according to the California Department of Public Health (CDPH).
The rate of illness -- 14.5 per 100,000 population -- is the highest since 1959 (16.1 per 100,000).
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Of the cases with hospitalization information, 10 percent required admission. Three-quarters of hospitalizations occurred in infants younger than 6 months, and of those, three-quarters were Hispanic.
Nine babies have died, including eight younger than 2 months -- the age at which pertussis vaccination starts -- and one 2-month-old who had been born prematurely and who had received just one dose of the tetanus, diphtheria, and pertussis vaccine (Tdap). Eight of the infants were Hispanic, which fits a trend of a disproportionate number of deaths among that ethnic group reaching back to 1998.
The first indication of an increase in pertussis cases in California was detected in April, and an epidemic was declared by CDPH director Dr. Mark Horton in June.
Since then, CDPH has been actively encouraging the public to get vaccinated. The agency expanded its recommendations for a booster dose -- typically administered around age 11 -- to comprise everyone 7 years and older who is not fully immunized, including adults of all ages. It specifically noted the importance of vaccination in women of reproductive age before, during, or immediately after pregnancy and in other people who have close contact with pregnant women or infants.
In addition to pushing vaccination for children and those around young infants to help slow the epidemic, CDPH has provided free vaccine to birthing hospitals, community health centers, Native American health centers, and local health departments, and has distributed educational materials to the public and clinical guidance to the medical community.
According to Dr. Mark Sawyer, a pediatric infectious disease specialist at the University of California San Diego, there have not yet been any signs that the epidemic is slowing, although he noted that pertussis activity typically dampens in the winter months.
Sawyer, who is a member of the Advisory Committee on Immunization Practices (ACIP), said in an interview that pertussis activity tends to be cyclic, peaking every two to five years, perhaps related to climate or to the build-up of susceptible individuals in the community.
According to CDPH, the last peak was in 2005, when there were 3,182 cases, including 574 hospitalizations and eight deaths.
There is no clear explanation for why pertussis broke out this year. Sawyer said parental refusal of vaccination probably plays a small part, but that a bigger problem is the older children and adults who do not get immunized because they do not know they should be.
Most children receive five doses of the Tdap vaccine before kindergarten starting at age 2 months, but protection wanes, requiring a booster that is required by most states before a child starts middle school. California just recently passed legislation implementing such a mandate that will go into effect next year.
Sawyer said it is unclear how long the booster dose lasts. Studies are ongoing and ACIP will be addressing the issue in upcoming meetings.