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NEW YORK - A study of nearly 190,000 young women injected with Merck & Co's human papillomavirus vaccine Gardasil found no new safety concerns.
Researchers said the only side effects they observed - rare cases of skin infections and fainting - were benign and expected.
"This analysis was very reassuring," said lead researcher Dr. Nicola Klein of the Kaiser Permanente Vaccine Study Center in Oakland, California, whose work was funded by Merck.
HPV vaccination is recommended for both boys and girls to shield them from the sexually transmitted infection, which may cause various types of cancer.
While Gardasil has already been deemed safe and effective by health regulators across the globe, large follow-up studies are typically required after vaccines hit the market to make sure less-common side effects haven't been missed.
Klein's team studied girls and women ages 9 to 26 who had had at least one Gardasil shot between 2006 and 2008. Based on electronic records, they compared ER visits and hospitalizations soon after vaccination and several months later, when short-term side effects would no longer be expected to show up.
Three doses of Gardasil, the recommended amount, cost about US$360 (S$440). US health regulators have found no serious side effects apart from soreness at the injection site.
At first glance, several health conditions appeared to be linked to the vaccination, including seizures, allergic reactions and birth defects in vaccine recipients.
A special safety committee without ties to Merck concluded that most of these conditions were not truly caused by the vaccine, however, but rather unrelated or preexisting conditions detected when the shot was given.
"All of a sudden these things get brought up again," said Klein, "but it has absolutely nothing to do with the vaccination."
After reviewing medical records for a number of different conditions that might be linked to the vaccine, the researchers determined that only two of them were legitimate findings.
In the two weeks after getting the vaccine, there were 1.8 cases of skin infections per 10,000 women, compared to just one case per two weeks per 10,000 women months away from the vaccination.
And some of those "infections" might simply have been red swellings at the injection site, said Klein, whose findings are published in the Archives of Pediatrics & Adolescent Medicine.
A stress reaction?
A STRESS REACTION?
On the day of the vaccination, the rate of fainting was six times higher than usual. But again, the risk was very low and has been seen before in other vaccine studies, Klein said.
It's possible that fainting may be a stress reaction to the injection itself, according to Dr. Michael Brady, an expert in pediatric infectious diseases at Nationwide Children's Hospital in Columbus, Ohio.
He said adolescents should sit down for 15 minutes after getting a shot, as also advised by health officials.
"If they do fall and hit their head it can have consequences, so it's not something to be ignored," added Brady, who was not involved in the new work and has no ties to HPV vaccine makers.
Overall, he said the study didn't bring up any unexpected concerns.
"The combination of a vaccine that is working and is safe should make people feel comfortable," Brady added.
Gardasil is expected to prevent more than 90 per cent of genital warts and between 60 and 70 per cent of cervical cancers, according to Dr. Christopher Harrison of Children's Mercy Hospitals and Clinics in Kansas City, Missouri.
"The risk benefit here is clearly and resoundingly in favour of the vaccine being much more beneficial than risky," said Harrison, who was not linked to the new study and has not had any relationship to Merck for the past three years.