Can breastfeeding transmit yellow fever after maternal vaccination?

A five-week old infant most likely contracted a vaccine strain of yellow fever virus through breastfeeding, according to a case report published in CMAJ (Canadian Medical Association Journal).

'Until recently, avoidance of vaccination of breastfeeding women with yellow fever vaccine had been based on theoretical grounds only,' writes Dr Susan Kuhn, with coauthors. 'We report the probable transmission of vaccine strain of yellow fever virus from a mother to her infant through breastfeeding,' which supports current recommendations for breastfeeding mothers to avoid the vaccine.

The yellow fever vaccine is a live-virus vaccine that has been used since the 1940s.

When the infant was 10 days old the mother received pre-travel advice and travel vaccinations, including one for yellow fever. Subsequently, they travelled to Venezuela for one week and breastfeeding was continued. The infant did not receive vaccinations.

'The previously healthy five-week old infant male presented to the hospital with a two-day history of fever and irritability,' write the authors. 'The day before his admission, he had been noted to have focal seizures on alternating sides.' Testing of the spinal fluid revealed evidence of recent infection with the yellow fever virus. Given that the travellers elected to stay in urban Venezuela where yellow fever is not known to be a risk, the authors concluded that the likely explanation was transmission of the yellow fever vaccine strain through breastfeeding.

The baby showed no sign of insect bites, had not been in contact with sick people, was not exposed to animals in Canada or elsewhere, had no history of herpes infections in family members and had not had any vaccinations prior to his symptoms.

'This probable case of yellow fever virus further supports the current recommendations for avoidance of yellow fever vaccination in lactating mothers of infants under nine months of age,' write the authors. 'While there may be situations in which the mother will have unavoidable and significant risk of yellow fever exposure, the risk to the infant due to maternal vaccination must be weighed against the risk of wild-type virus infection.'

The authors conclude that travelling women should adjust their plans to reduce or limit their risk of exposure or postpone their trip entirely until their infant is no longer breastfeeding or is old enough to be vaccinated.