It's not always easy to get to the root of an infection outbreak. Epidemiologists study infected people, contacts, and carefully examine where the infections happened and when. In the case of a 2012 outbreak of pertussis — whooping cough — in Oregon, scientists just published an analysis of how vaccination status affected when a child became infected during the outbreak.
One factor behind cases of childhood diseases for which effective vaccines exist is parents' deliberate delay or refusal to immunize their children. Unvaccinated children can develop the disease — pertussis, for example — if exposed to another child with the infection.
Healthcare professionals rely on what is called "herd immunity" to help prevent widespread outbreaks. Herd immunity protects unvaccinated people in the community because if a significant proportion of the population is immunized, an infection won't be able to spread to too many people.
The new analysis presents evidence that children who are not immunized not only become infected themselves, but may have a role in driving community-wide outbreaks.
Researchers Steve G. Robison and Juventila Liko, from the Immunization Program of the Oregon Health Authority in Portland, published their findings in the Journal of Pediatrics in April.
Whooping cough (named for the "whoop" sound an infected person makes trying to get a breath of air) attacks people of all ages but can be deadly in infants. About half the infants infected with pertussis end up in the hospital.
The disease is caused by a bacteria called Bordetella pertussis and is usually spread by the coughs or sneezes of an infected person. Once infected, the bacteria attach to cells lining the respiratory tract, cause the airways to swell, cause violent coughing, and make breathing very difficult.
Infected people are most contagious up to about two weeks after the cough begins, but the cough can last for up to 10 weeks, and the disease has been called the "100 day cough." Antibiotics can shorten the time people are contagious, but the best bet against getting the disease is to get vaccinated.
The vaccine is not 100% effective — no vaccine is — and if there are pertussis infections in the community, a vaccinated person still may contract the disease, but it will not be as bad as in unvaccinated people.
The pertussis vaccine is given as part of the DTaP vaccine, a combination vaccine that helps protect against diphtheria, tetanus, and pertussis. The Centers for Disease Control and Prevention (CDC) recommends five doses of DTaP at certain intervals before age six, then another dose at age 11 to 12. Adults should get a vaccination that contains tetanus and diphtheria protection every ten years, but reimmunization against pertussis is not usually recommended — despite the fact that no one knows how long vaccination protection against whooping cough lasts.
CDC estimates there are about 16 million cases of pertussis worldwide and about 195,000 deaths every year.
In 2012 (the year of the Oregon outbreak) authorities reported 48,277 cases of pertussis in the US, the highest number of cases since the 62,786 reported in 1955.
And that's precisely the reason we need to study that outbreak to find out how the infection got such a stronghold in communities where herd immunity should have kicked in.
The researchers analyzed data from cases of pertussis among children two months to 10 years old that occurred during the 2012 Oregon outbreak from January to November. By the time the outbreak was over, 351 children across 72 different zip codes had contracted pertussis, indicating the persistence and pervasiveness of the infections. Different pockets of outbreaks were analyzed to track how and to what kids the infection spread.
Fifty unvaccinated children contracted the disease, and the remaining cases occurred in undervaccinated children — those who did not receive the required number of boosters — and fully vaccinated children. There were two pertussis outbreak clusters.
The first cluster occurred in a single zip code area during weeks 11 and 12 of 2012 and involved five cases in young unvaccinated children. The second cluster was longer and more far-reaching. It occurred from week 16 to 31 of 2012 and covered 29 zip codes that included most of metropolitan Portland. This cluster included 39% of the cases in the 2012 outbreak.
As the outbreak spread, the earliest cases in newly affected areas were mostly unvaccinated. Young children who are not immunized tended to have pertussis one to two weeks before children who were immunized and lived in the same zip code. The researchers noted that these unvaccinated children were more frequently the source of the bacterial disease that was then passed on to others.
Even though 90% of children in Oregon are immunized against pertussis, this protection is not enough to stop the disease from spreading when the unvaccinated share social networks, schools or other connections, according to the study authors. They concluded that the unvaccinated have a greater role in how outbreaks develop beyond simply not helping build community protection.
"In the simplest epidemiologic sense, children who are not immunized count against the herd immunity levels needed to prevent the spread of disease," wrote the authors.
When some parents choose not to immunize their children, the consequences of their decisions have much further reaching implications than those directly affecting their child. Their decision may undermine the stability of herd immunity in communities and put all the people there at risk of contracting an otherwise preventable disease.