Pertussis Outbreaks on the Decline, but Immunization Gaps Still Exist

Outbreaks of pertussis ballooned in regions across the United States in 2012, breaking records and causing hospitalizations and deaths. According to the CDC, more than 41,000 cases were provisionally reported in 2012, along with 18 deaths. And although report numbers may ease in 2013, the threat of whooping cough remains significant.

The Minnesota Department of Health, which has set its preliminary estimate of 2012 pertussis cases at 4,485, said it already has seen 90 cases reported through Jan. 25 of this year.

In Vermont, which saw 645 cases reported in 2012 compared to 94 in 2011, 21 cases of pertussis have been confirmed through Jan. 19. In 2012, 27 cases were reported during the same period.

The Washington State Department of Health is reporting only 17 cases through Jan. 26 of this year versus 69 for the same period in 2012. In April 2012, the state declared a pertussis epidemic, eventually reporting 4,852 cases for the entire year.

The CDC currently is evaluating potential causes of the increase in whooping cough cases, including changes in disease-causing bacteria types. "Multiple types or strains of pertussis bacteria can be found causing disease at any given time, including during outbreaks," said the agency. "There is a lot of diversity in strains causing pertussis in the U.S."

According to the CDC, the recognized evolution of pertussis strains does not seem to be the cause of the 2012 pertussis spike. There also is no direct evidence that strain changes have reduced the effectiveness of the vaccines -- diphtheria and tetanus toxoids and acellular pertussis adsorbed (DTaP) and tetanus, diphtheria and acellular pertussis (Tdap) -- used to protect the population from such outbreaks.

"It's more likely that waning immunity is the driving force behind the increase in cases," the CDC said.

A Sept. 13, 2012, article (abstract) in the New England Journal of Medicine supported that stance, pointing out the relatively short-lived ability of the DTaP vaccine to fight off pertussis, as well as how DTaP's level of protection diminishes sharply during the five years after the fifth and final dose in the childhood series.

"The findings suggest that whooping cough control measures may need to be reconsidered," lead author Nicola Klein, M.D., Ph.D., said in a related news release. "Prevention of future outbreaks may be best achieved by developing new pertussis-containing vaccines or reformulating current vaccines to provide long-lasting immunity.

"That said, the DTaP vaccine is effective and remains an important tool for protection against whooping cough for children and the communities in which they live, and following current CDC recommendations remains important."

In an interview with AAFP News Now, CDC epidemiologist Stacey Martin, M.Sc., said that immunization advice will to have to suffice until vaccine modification becomes reality.

"There are vaccines in the works, but they are all at least a good decade away," Martin said. "So right now, I think we've got to use the vaccines we have the best way that we can."

The CDC and its Advisory Committee on Immunization Practices (ACIP) currently recommend that adolescents receive a booster Tdap dose at age 11-12, and children as young as 7 may receive a dose of Tdap under certain circumstances. In addition, in its just-released 2013 immunization schedules, the ACIP notes that all adults ages 65 and older should be offered a Tdap booster, and all pregnant women should receive Tdap in the late second or early third trimester of each pregnancy, regardless of their Tdap immunization history.

"Since our biggest concern always is infants too young to have any benefit from vaccination -- when we are seeing so much community transmission, really, a lot of our prevention efforts are going toward protecting those younger children," Martin said. "First, it was the initial pregnancy recommendation, then the cocooning recommendation (for other close contacts), and now, there's a recommendation for every pregnancy.

"That's probably the most effective means of protecting young infants, and while the evidence still is a bit limited, we are doing studies to try to get a better sense of what the real effectiveness is during those first few weeks of life when Mom's been vaccinated during pregnancy."

Martin said that increasing immunization coverage is a constant focus for the CDC, regardless of vaccine efficacy, because logic suggests that a better-vaccinated population will be less susceptible to infection.

"It is important that, while we know these vaccines do wane -- and that they may wane more quickly than we would like -- they're still our best prevention tool," she said. "And yes, we do see some breakthrough disease, but it is really important that parents get their kids vaccinated and that people get their Tdap booster on schedule."

Going forward, Martin said it is hard to predict the size or number of pertussis outbreaks in 2013.

"You typically see these peaks in disease and then see the susceptible population has dropped, and then it takes a few years for a new group of susceptibles to come through," she said. "And with this new 'waning early' factor we are seeing from the childhood series and, possibly, a short duration from the Tdap booster, we're really not sure what to expect in 2013."

Cynthia Kenyon, M.P.H., senior epidemiologist with the Minnesota Department of Health, said that 2013 would most likely see a drop in the number of pertussis cases reported in her state, but she admitted things could change.

"We've seen so much changing epidemiology in Minnesota that, going forward, we need to watch how this plays out," she told AAFP News Now. "I think the number comes down because we had so much disease that vaccine uptake rose accordingly, which means that our state- and community-wide immunity levels are probably pretty high. So I'd expect to see a lower number (of cases), but how much lower, I cannot be sure."