Inactivated Flu Vaccine Associated With Fewer Medical Visits for Respiratory Illness Than Intranasal Vaccine

A study among U.S. military personnel finds that those who received a flu shot with the trivalent inactivated vaccine had fewer subsequent healthcare visits related to pneumonia and influenza than those who received an intranasal live attenuated influenza vaccine, according to a study appearing in the March 4 issue of JAMA.

Military personnel are prone to outbreaks of respiratory illness such as influenza for a variety of reasons, including crowding and stressful conditions. Trivalent inactivated vaccine (TIV), administered intramuscularly, was first developed and tested in the military in the 1940s and has been used annually since the 1950s to prevent influenza and its complications. In 2003, a live attenuated influenza vaccine (LAIV) was formulated for intranasal application and approved for use among healthy adults, according to background information in the article. Service members were immediately targeted for LAIV use by the U.S. Department of Defense because of the ease of vaccine administration and availability early in the season. Since 2004, increasing numbers of military personnel have been immunized with LAIV while most others received TIV. However, data about live virus vaccine effectiveness among healthy adults are limited.

Zhong Wang, PhD, MPH, of the Armed Forces Health Surveillance Center in Silver Spring, Md., and colleagues investigated the incidence of healthcare encounters for pneumonia and influenza illness among active-duty service members, age 17 to 49 years, eligible for influenza vaccination who were stationed in the United States during the 2004-2005 (n = 1,061,728), 2005-2006 (n = 1,041,264), and 2006-2007 (n = 1,067,959) influenza seasons. Immunization rates ranged from 51.9 percent in the 2004-2005 to 78.4 percent in the 2006-2007 influenza season. The proportion of immunized persons receiving LAIV increased from 33.5 percent in the 2004-2005 influenza season to 47.9 percent in the 2006-2007 season.

The researchers found that the incidence rate of healthcare encounters for pneumonia and influenza was highest in the unimmunized group each season, with the LAIV immunized group having the next highest incidence rates, and the TIV immunized group with the lowest incidence.

The incidence rates of hospitalizations for pneumonia and influenza were highest in the LAIV immunized group for each of the three seasons, and the incidence rate in this group was significantly higher than that in the unimmunized group during the 2004-2005 season but not during 2005-2006 or 2006-2007.

Live attenuated influenza vaccine was found to have an effect similar to TIV in those who had not received a flu vaccine before. “This suggests that pre-existing vaccine-induced immunity may play a role in determining the effectiveness of LAIV,” the authors write.

“These results suggest that in a highly immunized adult population, TIV may be more effective than LAIV for the prevention of pneumonia- and influenza-related morbidity. Live attenuated influenza vaccine may be more appropriate for those with no prior immunization, such as military recruits,” the researchers write. “Because our population is highly immunized against influenza on an annual basis, results from this report may not be generalizable to the entire U.S. adult population but could be useful for nonmilitary adult populations where vaccinations rates are high. Additional efficacy trials in this population or effectiveness studies using laboratory-confirmed influenza infections may be warranted.”

Reference: JAMA. 2009;301[9]:945-953.