The researchers say their analysis showed that frequent colds, bacteria in the nose and not being breastfed were major risk factors for ear infections in babies.
The team - from the University of Texas Medical Branch (UTMB) at Galveston - reports the findings in the journal Pediatrics.
The researchers find that compared with similar studies from 20-30 years ago, rates of ear infection in American babies have fallen from 18% to 6% in 3-month-olds, from 39% to 23% in 6-month-olds, and from 62% to 46% in 1-year-olds.
They suggest use of vaccinations, higher rates of breastfeeding and a decline in smoking rates could be major reasons.
In the US, acute otitis media (AOM), or ear infection, is one of the most common infections in babies and young children.
Ear infection is also the biggest cause of childhood doctor visits and the most common reason children are prescribed antibiotics or have to undergo surgery, note the researchers.
There is evidence, they add, that repeatedly having ear infections up to the age of 6 months can lead to suffering the complaint more frequently later in life.
Lack of breastfeeding major risk factor
For their study, the team followed 367 babies from the age of under 1 month to their first birthday over the period 2008-2014. Altogether, the study covered 286 child-years.
The infants' parents informed the team whenever their child showed signs of ear infection or having a common cold (an upper respiratory infection, or URI). A doctor appointed by the team then examined the child within 5 days.
The researchers regularly took nose and throat mucus samples from the infants, and also whenever they had a cold. They analyzed the samples for presence of bacteria and viruses.
The team also collected information about family history of ear infection, exposure to cigarette smoke and whether the infants were breastfed.
During the study period, the authors documented 887 cases of URI (affecting 305 babies) and 180 cases of AOM episodes (143 babies - nearly half of the group).
Lead author Tasnee Chonmaitree, a professor in pediatrics, says their analysis clearly shows that frequent colds, having bacteria in the nose and not being breastfed were major risk factors for ear infections.
She notes that prolonged breastfeeding was linked to significant reductions in both colds and ear infections - which commonly develop from a cold in babies.
Breastfeeding provides babies with antibodies, including Immunoglobulin A (IgA), which helps to protect the mucous membranes from infection.
However, Prof. Chonmaitree also comments:
In addition, she and her colleagues note that interactions between bacteria and viruses may play a significant role in the development of ear infections and should be investigated further.
In September 2014, Medical News Today learned how another team, from Wake Forest Baptist Medical Center in Winston-Salem, NC, found that nasal infection with a flu virus can cause bacteria in the nose to travel to the middle ear.