Pertussis Resurgent in California
By Amesh A. Adalja, MD, July 2, 2010
Whooping cough, or pertussis, a disease thought by many to be of historical significance only, has steadily resurged and, in a few cases, has been deadly. Most recently, the California Department of Public Health declared a pertussis epidemic on June 23, 2010. So far this year there have been more than 1,500 confirmed and suspected cases and 5 deaths in the state—more than triple the figures for last year. Nationally, there have been 4,656 case of pertussis in the first 23 weeks of 2010, compared with 6,017 cases for all of 2009.1 Pertussis outbreaks tend to be cyclical, with epidemics occurring every 3 to 5 years. The last national epidemic occurred in 2005, when more than 25,000 cases were reported.2
Pertussis is Not a Benign Illness
That the California outbreak has already caused 5 deaths—all infants—is not surprising given that small children are prone to developing severe disease. The historical rate of infant hospitalization with pertussis is 63%, and the case-fatality rate among infants aged <2 months was 1.8% during the years 2000 to 2004.3 Adults with pertussis often suffer the consequences of coughing spells that are violent enough to fracture ribs.
Vaccination Policies Have Changed
Pertussis has been controlled chiefly through vaccination, with the pertussis series included in routine childhood immunizations and beginning at 2 months of age. In many locations, particularly California, vaccination rates have decreased because of parental concerns about the risks associated with vaccines. Some have speculated that the current outbreak may be related, at least in part, to decreasing vaccination rates. A large pertussis outbreak in England in the 1980s was associated with widespread refusal of vaccine.3
In 2007, recommendations were made to augment the immunization schedule with adult booster shots, to be included with routine tetanus-diphtheria boosters. This recommendation was made to address the waning immunity seen in adults, which renders them fully susceptible to pertussis and eligible to become vectors of disease spread to children.4,5
Herd Immunity Essential for Stemming Outbreaks
Because those at highest risk for pertussis—infants under 2 months of age—are not eligible for vaccine, it is imperative to create buffer zones of vaccinated individuals around them. Substituting the tetanus-diphtheria and pertussis (Tdap) booster shot for the routine tetanus-diphtheria (Td) booster is an important step in achieving high rates of adult immunization. Several hospitals prophylactically immunize parents of newborns prior to the baby’s hospital discharge to further protect vulnerable infants. Mathematical modeling studies have estimated that by vaccinating 90% of adolescent and adult household contacts of infants, 75% of infantile cases of pertussis can be prevented.6
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