US regulators have warned of a number of cases of potentially life-threatening twisting of the intestines in infants vaccinated against rotavirus gastroenteritis with a new treatment from Merck & Co.

The US Food and Drug Administration said it has received 28 post-marketing reports of intussusception following administration of Merck’s RotaTeq, although the agency points out that the number “that may have been caused by the vaccine, or occurred by coincidence, is unknown.”

Approximately half of the cases occurred 1-21 days after vaccination, and 16 of the 28 children with intussusception required surgery on their intestine. No deaths were reported and the FDA also noted that these figures are in line with the number expected in an unvaccinated population of infants. Approximately 3.5 million doses of RotaTeq have been distributed (though not necessarily administered) in the USA since its marketing licence was granted on February 3, 2006.

The FDA said it had issued the notification “both to encourage the reporting of any additional cases of intussusception that may have occurred or occur in the future after administration of RotaTeq,” and to remind people that the condition is a potential complication. Rotavirus is the leading cause of diarrhoea in infants worldwide, and severe cases cause as many as 70,000 hospitalisations and 60 deaths each year in the US alone.

In response, Merck said that it has updated the prescribing information for Rotateq to include post-marketing reports of intussusception and hematochezia (bleeding from the intestine), and noted that the former condition is “a naturally occurring event in infants,” estimated to occur in the USA in approximately 1 in 2,000 children during the first year of life.

Merck noted that in a pre-approval clinical trial of 70,000 infants, 35,000 infants received Rotateq (with the rest on placebo) and no significant increased risk of intussusception was found (six cases with the vaccine against five for placebo). It is currently conducting a post-marketing study of approximately 44,000 infants.

Merck is confident in the safety of its product, which had sales of $163 million last year, but will remember that intussusception was the reason why Wyeth's RotaShield vaccine against rotavirus was withdrawn in 1999.