Flu Vaccine Safe in Kids Six to 23 Months

DENVER, Oct. 24 -- The trivalent influenza vaccine appears to be safe in children between the ages of six and 23 months, according to researchers here.

In a review of more than 69,000 vaccinations among more than 45,000 children, "very few medically attended events (were) significantly associated with the vaccine," found Simon Hambidge, M.D., Ph.D., of Kaiser Permanente Colorado, and colleagues.

None of the events linked to the vaccine was serious, Dr. Hambidge and colleagues reported in the Oct. 25 issue of the Journal of the American Medical Association.

Flu vaccination has been recommended for all children ages six to 23 months since the winter of 2004-2005, but the largest previous study of its safety in these children included data on 8,476 vaccinations.

For this study, the researchers used a large cohort of children given the flu vaccine starting Jan. 1, 1991 - at a time when vaccination was recommended only for children at high risk of flu complications - and continuing to May 31, 2003.

The main outcome measure was any medically attended event significantly associated with the vaccine in four "risk windows" - zero through three days after vaccination, one through 14 days, one through 42 days, and 15 through 42 days - using ICD-9-CM codes.

The risk windows were compared with two control periods, one before vaccination and one after the risk windows, and the primary analysis looked at events in the period from one through 14 days after vaccination, Dr. Hambidge and colleagues said.

If a medically attended event was significantly associated with the vaccine, the researchers then conducted a chart review.

Of the 45,356 children included in the study, 16,536 had medical conditions that put them at higher risk for flu complications and the rest were healthy children.

The study found 14 events significantly more or less likely to occur within 14-day risk window, compared to both control periods, but only one - gastritis/duodenitis - was positively associated with the vaccine.

After chart review to exclude any events that were clearly not due to the vaccine, gastritis/duodenitis was no longer significantly associated with the vaccine, the researchers found.

On the other hand, on the basis of the ICD-9 codes, several conditions - such as otitis media, asthma, cough, and pneumonia - were less likely to occur in the risk windows than in both control periods.

Again, however, the significance did not survive chart review, the researchers found.

Two types of events remained significantly associated with the vaccine compared to the first control period, however, although not to the second:

• Convulsions treated in the emergency department were twice as likely in the 14-day risk window than in the period before vaccination. The odds ratio was 2.0, with a 95% confidence interval from 1.0 to 4.0.
• Non-infectious gastroenteritis, treated in the emergency department, was nearly twice as likely in the 14-day risk window than in the period before vaccination. The odds ratio was 1.93, with a 95% confidence interval from 1.01 to 6.68.

The gastroenteritis was unexpected, the researchers said, and may be due to chance, although "we cannot rule out that it could be a reaction to the vaccine."

"Fortunately, this diagnosis, even if due to trivalent inactivated influenza vaccine, is generally mild and self-limited," they commented.

Most of the convulsions were febrile and they peaked in the second week after vaccination, a time period not thought to be compatible with a reaction to a formalin-inactivated vaccine, such as the trivalent influenza vaccine, Dr. Hambidge and colleagues said.

On the other hand, the MMR vaccine is known to have an increased risk for febrile convulsions in that time frame. When the researchers excluded children who had a concurrent MMR vaccination, the febrile convulsions were no longer associated with the trivalent vaccine.

"Thus, it appears we were detecting the known association of MMR and febrile seizures," Dr. Hambidge and colleagues concluded.