The pertussis vaccine, which was first introduced in combination with diphtheria and tetanus vaccines in the 1940s, consisted of whole pertussis bacteria inactivated with formaldehyde. This vaccine, which contained about 3,000 structural and non-structural proteins from the bacteria as well as inactivated pertussis toxins (i.e., toxoids), was phased out in the late 1990s in favor of a vaccine containing only two to five inactivated pertussis proteins (the so-called acellular vaccine). The result was a much better safety profile. But, as we’re learning over the past few years, the switch has also resulted in a decrease in vaccine efficacy.
Recently, Nicola Klein and colleagues at Kaiser Permanente in Northern California investigated the effectiveness of the Tdap adolescent booster during a pertussis epidemic in 2014 (Klein N, Bartlett J, Fireman B, Baxter R. Waning Tdap Effectiveness in Adolescents. Pediatrics doi: 10.1542/peds.2015-3326). What they found was a little disheartening. Tdap effectiveness steadily waned within a few years. Vaccine effectiveness one year after administration of Tdap was 68.8 percent; after two years, 56.9 percent; after three years, 25.2 percent; and after four or more years, 8.9 percent. The authors concluded that, “adolescents who were more remote from Tdap were significantly more likely to test positive than were those vaccinated more recently.” Consistent with this statement, these same investigators had previously shown during an outbreak of pertussis in California in 2010 that children who had received the pertussis vaccine were eightfold less likely to get pertussis than those who hadn’t.
I think it’s fair to say that we would benefit from a better pertussis vaccine. However, although the pertussis vaccine is imperfect, it’s still the best tool we have to fight this infection.