The Tdap vaccine – tetanus, diphtheria and acellular pertussis – given to preteens as a booster loses effectiveness over time, according to new study from the Centers for Disease Control and Prevention (CDC). The research, published online May 4 in the journal Pediatrics, suggests this waning immunity may be the reason for the recent uptick in the number of whooping cough (pertussis) cases in the U.S.
“Among adolescents, within the first year following immunization the vaccine effectiveness was 73 percent,” study author Anna Acosta, MD, a medical epidemiologist in the CDC’s division of bacterial diseases, told HealthDay. “But by two to four years out, it had fallen to about 34 percent.”
A highly contagious respiratory tract infection, whooping cough can cause vomiting, interfere with breathing and lead to extreme fatigue. It is especially dangerous to infants, putting them at higher risk for severe illness and even death. According to Acosta, the risks are less severe for teens, but still “significant.”
The staggering rise in whooping cough cases in the U.S. — 48,000 in 2012; the most since 1955 — reignited concern among the scientific community that the acellular pertussis vaccine that replaced the whole-cell vaccine in 1997 because of a high number of reactions, was losing immunity strength. In 2005, that concern led to the addition of the Tdap vaccine for kids aged 11 and 12 and for adults who had never been immunized.
For the study, Acosta and her team focused on pertussis cases that reached epidemic proportions in Washington state in 2012 when more than 5,000 people came down with whooping cough. Many of the ill were in their early teens, despite the state’s 86 percent Tdap vaccination rate.
The research team looked at nearly 1,700 teens born between 1993 and 2000, the years when children were more likely to have received the acellular vaccine. They then analyzed the records for all pertussis patients reported from Jan. 1, 2012 to June 30, 2012 in the seven Washington state counties with the highest number of cases.
The team found that in the 450 teens who got pertussis during the outbreak — compared with the 1,246 who didn’t get sick — the overall vaccine effectiveness was 64 percent. After the first year it is was 73 percent, but it fell to 34 percent in years two to four post-vaccination.
The new study appears to confirm what others have suggested – that the switch from the whole-cell pertussis vaccine to acellular types impacted vaccine efficacy.
“We do have a safer vaccine, and it’s a vaccine that works, but it doesn’t work for very long,” Ed Marcuse, MD, professor emeritus of pediatrics and vaccine expert at the University of Washington, told The Seattle Times.
However, cautioned Marcuse, who was not involved in the study, the vaccine is better than none at all. “While we’re seeing more cases now than when we had whole-cell vaccine, we’re seeing far fewer cases than when we didn’t have a vaccine,” he said.
Despite the waning effectiveness of the Tdap, Acosta and other health officials still see the vaccine as important and recommend that families follow CDC guidelines for the series of shots. “We immunize against pertussis to protect babies,” explained Marcuse. “Babies under three months of age die from this disease.”
In addition, the team stressed that expectant mothers should continue to get the vaccine during their third trimester of each pregnancy. Maternal inoculation “will protect the infant until they are able to receive their own doses at 2, 4, and 6 months of age,” Acosta told HealthDay.