Infants are now most likely to catch whooping cough from their brothers and sisters, a new study says.
"Knowing where they're getting their disease from is important so we can target our approach accordingly," said lead author Tami Skoff, of the Centres for Disease Control and Prevention (CDC) in Atlanta.
Whooping cough, which gets its name from the sound patients make while gasping for air, can be especially dangerous - and sometimes deadly - for newborns, according to the CDC.
The disease is more formally known as pertussis.
In the past, mothers were thought to be the most common sources of the infection for infants.
For the new study, the researchers used data collected between 2006 and 2013 from 1,306 infants in seven states. About a quarter were younger than two months.
By asking who else had whooping cough roughly one to three weeks before the infant's cough began, the researchers were able to figure out the source of the infection in 44 per cent of the infants.
When the source of infection could be identified, it was an immediate family member two thirds of the time. Siblings were the source of infection in 36 per cent of cases, while mothers were the source in 21 per cent and fathers in 10 per cent of cases.
Mothers were the main source of infection for infants until about 2008, according the researchers. Then, the main source became brothers and sisters.
The researchers write in Pediatrics that the change isn't surprising, because whooping cough is becoming common in older children. Partly, that's because protection from the newer versions of the whooping cough vaccine becomes weaker over several years.
So, "it makes sense that we're seeing this transition from mothers to siblings as the source of infection," Skoff told Reuters Health.
She also said the research suggests "cocooning," which is when people around the infant are vaccinated, is likely not effective - especially since her team could find a source of infection less than half the time.
Instead, she said, women should receive the whooping cough vaccine during each pregnancy to pass antibodies to the foetus, and those antibodies can protect the child until he or she is old enough to be vaccinated.
"You're providing direct protection to the mom and direct protection to the infant," Skoff said.
"There are some data out of the UK showing that vaccination during pregnancy is effective," she said. "The early data is very reassuring and promising, which is why we're pushing this strategy."