Flu Shots Not As Effective As Touted

A number of studies have suggested a 50% mortality reduction in older patients given flu shots. Some experts have questioned these figures as implausible. One explanation might be that those persons least likely to dies are also those most likely to get a shot; this is known as the “healthy-user effect”. To clarify this further, Canadian researchers have published the results of their study in the American Journal of Respiratory and Critical Care Medicine.

Another explanation is that vaccination against the influenza virus has a pleiotropic effect – meaning it “produces more than one effect”; thus in addition to its prevention of influenza (and thus its complications) it can reduce mortality from community-acquired pneumonia (CAP). To isolate such a benefit, the researchers studied people admitted to hospital with CAP outside of the flu season.

Full data were collected on 1,800-plus CAP patients admitted to 6 Canadian hospitals. Amongst them, 352 had received flu shots, and they were carefully matched with data from 352 other CAP patients who hadn’t had shots, and acted as controls. The mortality rate for those who had received a flu shot was half that of the non-vaccinated patients – 8% vs. 15%. This degree of benefit shown by the flu shots was not substantially altered after allowances were made for factors such as age, gender, nursing home residence, accompanying illnesses, and number of medications being taken. But when the researchers made additional allowances for functional status, socioeconomic status, and similar information, the 51% reduction in deaths was brought down to a 19% reduction. The principal investigator believes that, in fact, other similar factors, which they couldn’t assess from the hospital databases, were responsible for the differences achieved in vaccinated patients.

The general conclusion from this study is that the “healthy-user effect” is responsible for the apparent reduction in CAP deaths. So maybe a flu shot isn’t quite as necessary as we’ve been led to believe. Nevertheless, influenza is unpleasant enough to drive me to the flu shot clinic each year!