2 chickenpox shots may be better: study

Children given two separate vaccinations against chickenpox may be better protected against the disease than those given just a single needle, say U.S. researchers, who found that immunity following one dose may wear off over time.

The findings are of particular interest in Canada, where single-dose varicella vaccination is the norm. The United States recently ramped up the recommended number of shots to one at age 12 to 15 months and a second one at four to six years old.

'We saw that with longer time since vaccination, people were more likely to have disease that was not mild.' Study co-author Dr. Jane Seward, CDC

"Our data indicate there is some waning of vaccine-induced immunity over time," said study co-author Dr. Jane Seward, acting deputy director of the division of viral diseases at the Centers for Disease Control and Prevention (CDC).

The study,published in Thursday's New England Journal of Medicine,looked at data on 350,000 subjects between 1995 and 2004, of which more than 11,000 developed chickenpox. Of those, almost 1,100 got the illness despite being vaccinated (called "breakthrough disease").

Focusing on children aged eight to 12, the researchers found that children vaccinated five or more years earlier were 2.6 times more likely to have moderate to severe chickenpox than those inoculated less than five years before.

"We saw that with longer time since vaccination, people were more likely to have disease that was not mild," Seward said Wednesday from Atlanta.

She said the annual rate of breakthrough disease also rose significantly as the time since vaccination increased, meaning that a higher proportion of those vaccinated ended up getting sick.

A single dose of varicella vaccine confers about 85 per cent protection, and since its widespread introduction more than a decade ago the incidence of kids being hospitalized or dying from complications has plummeted, Seward said.

But the study suggests a second dose could boost immunity even higher, further limiting the number of cases and the severity of disease.

"That second dose of varicella vaccine could provide increased protection against waning of immunity," she said.

Still, the findings aren't the last word on ideal dosage, she cautioned, stressing that other populations need to be studied to see if the CDC results can be confirmed.