Tuesday, October 28, 2008

Flu Vaccination Investigated In PLoS Medicine

With the new flu season beginning, the open-access journal PLoS Medicine has released two studies related to influenza vaccination, each showing that the population burden of the flu can be decreased overall by increasing the number of people vaccinated. The flu vaccine is often recommended for specific groups of people who are at a higher risk of spreading or suffering from influenza in targeted vaccination programs. However, in 2000, the province of Ontario, Canada began a universal immunization program, promoting vaccination in and providing vaccination free of charge to all people over 6 months old. Jeff Kwong from the Institute for Clinical Evaluative Sciences in Toronto and colleagues examined this program's effects on influenza-related health outcomes. To do this, they examined data from both national and provincial records between 1997 and 2004 in order to compare Ontario's flu outcomes before and after the introduction of the universal vaccination program, as well as to compare Ontario's outcomes to those of other provinces which continued their targeted vaccination programs. In the analysis the group found that the universal vaccination program was associated with reductions in influenza outcomes, such as flu-related deaths, hospitalizations, and emergency room and doctor visits. The results also indicated that increasing immunization rates was not as clearly associated with a reduction in mortality and health care need in older people, especially older than 75 years, in comparison with younger people. Additionally, even with the better access to free flu vaccines in Ontario, a modest average of 38% of the overall household population reported that they had received the vaccinations. This suggests that immunizing younger people who come in contact with older people may still be useful in prevention of influenza-related outcomes. .The second study investigated this idea mathematically, capitalizing on a concept known as heard immunity, in which immunization of a few individuals in the population contributes to reduce overall exposure to people who are not immunized. Carline van den Dool and colleagues at the University Medical Center, Utrecht, the Netherlands created a mathematical model simulating the spread of influenza in nursing homes. Specifically, they found that by increasing the proportion of vaccinated health care workers from zero to 100% in a 30 bed nursing home department could reduce infections in patients approximately 60%. On average, vaccinating seven health care workers prevented one patient from influenza. Unfortunately, they found that even full health care worker vaccination does not guarantee complete herd immunity. This suggests that even when immunization rates are high, health care workers should be vaccinated each year to reduce risk to patients. The authors point out that one of the major drawbacks of the study was random variation which limits the abilities of small vaccine trials to assess the real relationship between vaccination and influenza. As a result, further studies should be performed to confirm these results. Cécile Viboud and Mark Miller of the Fogarty International Center, US National Institutes of Health, contributed an accompanying comment in which they discuss the merits and drawbacks of the observational study design for evaluation of influenza vaccination.

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