The second generation vaccines against rotavirus, the most common cause of severe childhood diarrhea worldwide, are associated with a small increased risk of a rare intestinal disorder called intussusception that may require surgery, two US studies suggested Tuesday.

However, the benefits of the vaccines, Merck's RotaTeq (RV5) and GlaxoSmithKline's Rotarix (RV1), outweigh this possible small risk, according to the two papers published online by the New England Journal of Medicine.

Rotavirus infection causes an estimated 527,000 deaths in children less than five years of age each year, according to the World Health Organization (WHO).

The first licensed rotavirus vaccine known as RotaShield, however, was forced to withdraw in 1999, only one year after licensure in the US, because of increased risk of intussusception.

RotaTeq and Rotarix were subsequently licensed, one in 2006, the other in 2008. The two are now recommended by the WHO for immunization of children worldwide.

Both vaccines have shown tremendous health benefits. For example, before they were available in the US in 2006, rotavirus was responsible for more than 200,000 emergency room visits and 55, 000 to 70,000 hospitalizations of young children each year. Also, 20 to 60 children died each year. But since 2006, hospitalizations and emergency room visits have decreased by more than 80 percent among immunized children.

But in a seven-year study of RotaTeq, researchers from Harvard Medical School and Harvard Pilgrim Health Care Institute looked at data from 507,874 first doses and 1,277,556 total doses and found an excess risk of 1.5 cases per 100,000 recipients after the first dose, which was roughly one tenth the risk associated with the first-generation vaccine.

However, no significant increase in risk was seen after the second or third dose.

In a five-year of Rotarix, researchers from the US Centers for Disease Control and Prevention (CDC) looked at 207,955 doses, including 115,908 first doses and 92,047 second doses, and identified 5.3 extra cases of intussusception within seven days after the administration of either dose. Only 0.72 cases would be expected per 100,000 unvaccinated young children.

In an accompanying editorial, Roger Glass of the National Institutes of Health and Umesh Parashar of the CDC wrote that the results of the studies should be interpreted with caution.

"What, then, is the message for the physician or nurse who administers rotavirus vaccines, and what is the implication for vaccine policy in developed countries?" the two asked.

"Certainly, the abundance of evidence in the United States and beyond indicates that intussusception can occur as a result of vaccination with either RV5 or RV1, but the risk is low, on the order of approximately 1 to 5 cases per 100,000 infants, with wide confidence limits."

"Many questions remain to be resolved ... (however,) the public health benefits of rotavirus vaccines ... are likely to be substantial and outweigh a small risk of intussusception," they added.