A new study published in the journal Pediatrics has identified in a shift in the way pertussis, commonly known as whooping cough, is transmitted, revealing that infants are most likely to catch the condition from their siblings.

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Researchers found that more than 35% of whooping cough cases are now passed on by siblings.

Whooping cough is a highly contagious respiratory disease affecting all ages, but particularly infants and children.

It can cause not only discomfort, but prolonged illness, complications and even death, especially in babies under a year old. In the US, about half the babies who contract the disease at this age are hospitalized.

In the new study, the researchers – including Tami H. Skoff of the Centers for Disease Control and Prevention (CDC) – identified 1,306 cases of infant pertussis between 2006 and 2013.

The new report reveals that while mothers have historically been the most likely source of infection, a shifting trend means that siblings are now more likely to pass on the infection.

The disease initially resembles a cold, with catarrh, runny nose and possibly a mild cough. Babies may show an irregular breathing pattern, called apnea. The incubation period is normally 5-10 days, but can be up to 3 weeks. Fits of violent and rapid coughing then follow, causing sharp inhalation with a “whoop” sound that gives the disease its name. This can be exhausting and may also cause vomiting.

The disease can last up to 10 weeks, and sometimes more. In some cultures, it is known as the “100-day cough.” Recovery can take time, with bouts of coughing reappearing intermittently for months, and periods in between when it seems that the disease has gone. Complications include pneumonia and broken ribs.

In their study, Skoff and colleagues found a change in transmission patterns that raises concerns about protection against pertussis.

Fast facts about whooping cough
  • Around 23% of whooping cough cases lead to pneumonia
  • 0.4% of cases lead to encephalopathy
  • 1.6% of cases lead to death.

Learn more about whooping cough

The findings reveal that 66.6% of cases are passed on by immediate family members, of which 35.5% now originate from siblings, with mothers accounting for only 20.6% and fathers accounting for just 10%.

The authors believe the shift reflects the “evolving epidemiology of pertussis in the United States.” They cite “waning immunity from pertussis vaccines among children and adolescents” as a contributing factor.

Infants and children are usually inoculated against pertussis through the Diptheria, Tetanus and Pertussis (DTaP) vaccine, regularly given to pre-teens, but protection decreases over time.

The Tetanus, Diptheria and Pertussis (TDaP) vaccine is a booster that can be given once every 10 years for teens and adults.

The researchers call for increased protection, especially for “young infants in the first few months of life before immunizations begin,” and for more focus on TDaP vaccination coverage, specifically during pregnancy, to protect very young babies.

While this may raise concerns in some circles, the CDC describe the whooping cough vaccine as “very safe” for pregnant women and their unborn babies, and say it is “very important” that the vaccine is administered during the last 3 months of each pregnancy, insisting that it will not increase the risk of complications.

In November 2014, Medical News Today reported on findings that TDaP is not associated with preterm delivery or small birth size.