Almost one in five neonates has a respiratory virus in their first month of life but many fail to show signs of illness, new research shows.

A University of Queensland study of 157 full-term infants found the common cold virus was the virus most detected, accounting for more than 70 per cent of all positive swabs.

The study detected 43 virus-positive swabs from a total of 574 swabs in 29 babies, with a virus detected in one baby who was just two days old.

The community-based birth cohort study collected weekly nasal swabs and recorded daily symptoms for newborns up to 28 days post birth between 2010 and 2012.

Researcher and PhD candidate Minda Sarna, of the UQ Child Health Research Centre, said almost half of the newborns did not develop any signs of illness, and as a result their virus detection often went unrecognised.

Ms Sarna said the high proportion of babies not exhibiting symptoms may be in part due to breastfeeding, and the protective antibodies contained in breast milk.

“We did have quite a high proportion of breastfed babies in our cohort and we have hypothesised that there’s such a high proportion of asymptomatic detection because they’re breastfed,” she said.

“It would also be partially because of the passive transfer of maternal antibodies across the placental barrier.”

The study, published in The Pediatric Infectious Disease Journal, found when parents reported symptoms they ranged from nasal discharge and congestion to dry coughs, wheezing and fever, while there was one instance of an ear infection.

The earliest detection of viruses in babies who went on to develop symptoms were also in the first week of life - on days six and seven.

In the study, parents sought medical advice for mostly minor symptoms in six cases, while no baby was admitted to hospital.

Ms Sarna said newborns with older siblings were more likely to have a virus detection, possibly transmitted from their brother or sister.

“We did find that infants had a higher risk of virus detection if they were not first borns. Hand-washing of siblings when they're handling their newborn sibling would limit this exposure.”

Ms Sarna said while high-risk birth cohort studies have found a close link between early exposure to respiratory viruses in infants and the development of asthma, respiratory illnesses are not unusual in healthy babies, and are often mild and self-limiting.

“We were a little bit surprised by how early the infants were virus positive but respiratory infections are really common and they do occur,” she said.

“Whether asymptomatic infections have any lasting impact is still a topic of research.”