The Centers for Disease Control and Prevention suggests that recurring pertussis outbreaks may be the “new normal.” Such outbreaks show that some of what we “know” about pertussis is still correct, but some things are evolving. So in this new year, what do we need to know about patient vulnerability post vaccine as well as the clinical course, diagnosis, and treatment of this stubborn persisting disease?
Vulnerability after acellular pertussis vaccine
The recent large 2014 California outbreak surpassed the record numbers for the previously highest incidence year, 2010 (MMWR 2014;63:1129-32). This is scary because more cases had been reported in California in 2010 than in any prior year since the 1940s. The overall 2014 California pertussis rate (26/100,000 population) was approximately 10 times the national average for the first 9 years of this century, Are there clues as to who is most vulnerable and why?
No age group was spared, but certain age groups did appear more vulnerable. Infants had a startling 174.6/100,000 incidence (six times the rate for the overall population). It is not surprising to any clinician that infants less than 1 year of age were hardest hit. Infants have the most evident symptoms with pertussis. Infants also have 5-7 months of their first year in which they are incompletely immunized. Therefore, many are not expected to be protected until about 7-9 months of age. This vulnerability could be partly remedied if all pregnant women got Tdap boosters as recommended during pregnancy.
Of note, 15-year-olds had an incidence similar to that of infants (137.8/100,000). Ethnically, non-Hispanic whites had the highest incidence among adolescents (166.2/100,000), compared with Hispanics (64.2/100,000), Asian/Pacific Islanders (43.9/100,000), and non-Hispanic blacks (23.7/100,000). Disturbingly, 87% of cases among 15-year-olds had received a prior Tdap booster dose (median time since booster Tdap = 3 years, range = 0-7 years). Previous data from the 2010 outbreak suggested that immunity to pertussis wanes 3-4 years after receipt of the last acellular pertussis (aP)–containing vaccine. This is likely part of the explanation in 2014 as well. However, waning immunity after the booster does not explain why non-Hispanic whites had two to six times the incidence of other ethnicities. Non-Hispanic whites are thought to be the demographic with the most vaccine refusal and vaccine delay in California, so this may partially explain excess cases. Racial differences in access to care or genetic differences in disease susceptibility also may play a role.
Why is this biphasic increase in incidence in California a microcosm of the new epidemiology of pertussis in the United States? A kinder, gentler DTaP vaccine replaced the whole-cell DTP in the late 1990s. This occurred in response to the public’s concern about potential central nervous system adverse effects associated with the whole-cell DTP vaccine. Immunogenicity studies seemed to show equivalent immune responses in infants and toddlers receiving DTaP, compared with those who received DTP. It has only been in the last 5 years that we now know that the new DTaP and Tdap are not working as well as we had hoped.
The two aspects to the lesser protection provided by aP vaccines are pertactin-deficient pertussis strains and quicker waning of aP vaccine–induced immunity. Antibody to pertactin appears to be important in protection against clinical pertussis. New circulating clinical strains of pertussis may not have pertactin (N. Engl. J. Med. 2013;368:583-4). The strains used in our current DTaP and Tdap were designed to protect against pertactin-containing strains and were tested for this. This means that a proportion of the antibodies induced by vaccine strains are not useful against pertactin-deficient strains. The aP vaccine still induces antibody to the pertussis toxin and other pertussis components in the vaccines, so they will likely still reduce the severity of disease. But the vaccines are not likely to prevent infections from pertactin-deficient strains. This is similar to the partial vaccine mismatch that we are seeing with the current seasonal H3N2 influenza vaccine strain.
The second aspect is that protection appears to wane approximately 3-5 years after the last dose of aP-containing vaccine. This contrasts sharply with expectations in the past of 7-10 years of protection from whole cell pertussis–containing vaccines. The less reactive aP vaccine produces fewer adverse effects by not producing as much inflammation as DPT. The problem is that part of the reason the DPT has such good protective responses is the amount of inflammation it produces. So with less aP vaccine–induced inflammation comes less robust antibody and T-cell responses.
Nevertheless, the current acellular pertussis vaccines remain the most effective available tools to reduce pertussis disease (Cochrane Database Syst. Rev. 2014;9:CD001478]). But until we have new versions of pertussis vaccines that address these two issues, we clinicians need to remain vigilant for signs and symptoms of pertussis.