CHICAGO » Infants are more likely to catch whooping cough from their siblings than their moms, says a study with implications for how to protect them.

Previously, moms were considered the more common source of infection. The change is probably due in part to an increased frequency of whooping cough among school-aged children, the study of government data suggests.

Infants face the greatest risk of severe illness and death from whooping cough, especially before 2 months, the recommended age for beginning a series of vaccinations against the disease, the study notes.

To protect infants, experts have recommended two strategies — vaccinating close contacts after birth, including parents and siblings, and vaccinating pregnant women, who can transfer some disease protection before birth.

The study results suggest the first option may not be as effective as hoped, so boosting low vaccination rates among pregnant women should be a top priority, the study authors said.

The research examined data from 1,306 infants in seven states who were diagnosed between 2006 and 2013. The source of infection could be identified for about half the children. Of these, siblings were linked to more than a third of the cases and moms to about one-fifth. Dads accounted for another 10 percent, a bit higher than grandparents and aunts or uncles.

Results were published online Monday in the journal Pediatrics.

Whooping cough, also called pertussis, is highly contagious and typically begins with cold-like symptoms followed by severe coughing fits, although not all infants and young children develop a cough. Five vaccine doses are recommended by age 6, with a booster shot at age 11 or 12. Booster shots also are recommended for adults.

Vaccination rates are high for U.S. children and teens. The shots provide good protection for a few years, and although effectiveness tends to wear off, kids should continue to get vaccinated, said lead author Tami Skoff, a researcher at the federal Centers for Disease Control and Prevention.

But until vaccines with longer-lasting immunity are developed, getting more women immunized during every pregnancy is the best way to protect infants, she said.

Only 15 percent to 20 percent of U.S. pregnant women get whooping cough shots, she noted. Women who develop whooping cough during pregnancy can’t transmit the disease to fetuses, but transmission is possible if mothers cough during childbirth, Skoff said, "so the benefits of maternal vaccination during pregnancy are two-fold: protect both the mom and the infant."

Outbreaks of whooping cough are common and cases spike every few years. Last year, nearly 29,000 cases were reported and eight infants died, versus nearly 49,000 cases in 2012 — a peak year when 16 infants died, most of them younger than 3 months.