Infants are more likely to become infected with pertussis, or whooping cough, from their siblings than from their mothers, according to a new study from the Centers for Disease Control and Prevention (CDC).
This finding marks a major shift from the past, when mothers were believed to be the most common source of transmission to infants of this highly infectious bacterial disease.
The study underscores the need for pregnant women to get a pertussis booster during their third trimester, say CDC officials. The antibodies created in the mother’s body by the vaccine are passed on to her baby, offering the child some protection from the disease until he or she is old enough to be vaccinated.
It also underscores the need for everybody who is around a newborn to make sure they, too, have received a pertussis booster vaccine.
The study was published earlier this week in the journal Pediatrics.
For the study, researchers analyzed data from seven Enhanced Pertussis Surveillance sites around the country, including Minnesota. They identified 1,306 infant cases of pertussis at those sites between 2006 and 2013.
About a quarter of these cases (24 percent) involved children less than two months old. About a third of the infants (34 percent) were hospitalized because of the infection, 15 percent developed pneumonia and two (less than 1 percent) died.
The researchers could identify the likely source of the infection in only 44 percent of the cases. In those cases, about 85 percent of the infants became infected through a family member. The most common route was through a brother or sister (35 percent), followed by a mother (21 percent), a dad (10 percent), a grandparent (8 percent) and aunts or uncles (6 percent).
“Our study revealed the emergence of siblings as the major reservoir of infection and represents an important shift in the dynamics of pertussis transmission to young infants in the United States,” the researchers conclude.
The study’s main finding — that siblings are now the leading source within families of the transmission of whooping cough to infants — “is not that surprising,” said Cynthia Kenyon, a senior epidemiologist at the Minnesota Department of Health (MDH) and a co-author of the study.
Around 2008, she explained, public health officials began to notice that more 10- and 11-year-olds were getting pertussis than in the past. Those children were the first to have received only the acellular pertussis vaccine (DtaP) rather than the older “whole-cell” version (DPT). (Both formulations also include vaccines for diphtheria and tetanus.)
“The difference between DtaP and the older vaccine is that it begins to wane about two years earlier,” Kenyon said.
A booster vaccine (known as Tdap) has been recommended since 2005 for adolescents (and adults), but the earlier waning immunity of DtaP increases the possibility that a child will become infected with pertussis and pass it on to a newborn sibling before receiving the booster.
Indeed, the new study found that the median age of the siblings who transmitted pertussis to infants in their families was 8 years — the age when the effectiveness of the DtaP vaccine can begin to wane.
Pertussis is spread through droplets produced during coughing or sneezing. Among family members or others living in the same household, the disease has an infection rate of about 80 percent.
The danger for infants
The DTaP vaccine is not recommended for infants younger than two months. That leaves such children vulnerable to the disease. Pertussis can be a very serious illness, especially in infants, leading to pneumonia (lung infections), difficulty breathing and death.
In 2013, an otherwise healthy 1-month-old infant died in Minnesota after becoming infected with the disease, said Kenyon. Health officials believe pertussis was most likely transmitted to the infant from another child living in the household.
In 2014, 52 children under the age of 12 months were diagnosed with pertussis in Minnesota. Nine were hospitalized, but fortunately none died.
The new study’s findings point out the limitations of the current pertussis vaccine, with its much shorter period of effectiveness. Pertussis, which was almost wiped out during the 1970s, has made a dramatic comeback in recent years.
More than 28,000 cases of whooping cough occurred in the United States in 2014, including 949 in Minnesota. This year, 375 confirmed or suspected cases have been reported in Minnesota (as of Aug. 27.)
Ironically, use of the older DPT vaccine was phased out because vaccine critics said its rate of side effects — particularly its potential for causing fevers that, in rare cases, triggered seizures — was too high.
Bringing back the older vaccine is not, however, an option, said Kenyon. The vaccine is no longer being made in the United States, and it’s unlikely that parents would accept its reintroduction due to its higher rate of reactive fevers.
“What we need to do — while we do research to find a vaccine that lasts longer — is to focus on effective strategies that will protect our most vulnerable population,” she said.
That means educating the public and health-care providers about the need for pregnant women — and everybody else — to be up to date with their whooping cough vaccine.
Many older children and adults can have pertussis without knowing it, Kenyon stressed. They may have a lingering pertussis-related cough that they mistakenly attribute to a cold or flu. In addition, some evidence suggests that the certain people can be carriers of the disease without exhibiting any symptoms.
“My take-home message is this,” she said. “If you’re pregnant, it’s really important that you get the vaccine. It’s also important for other adults to make sure they’re up-to-date with Tdap. And if you have a prolonged cough, go to your provider and get tested for pertussis.”