Flu vaccine benefits in kids
New vaccines only get approved after randomized controlled trials showing that they are beneficial, but if you want to estimate the benefit of expanding vaccination to a new group of people it’s hard to do a randomized trial. Just comparing vaccinated and unvaccinated people doesn’t help, since there are many reasons why these groups are different, and the comparison gives completely unreasonable estimates. There’s a new study from Canada, reported by Reuters, that takes advantage of a ‘natural experiment’ to estimate the benefit of vaccinating children aged 2-4.
In the US, the guidelines on vaccination changed in 2006 to include kids in this age range, in Canada the guidelines didn’t change until last year. This allowed the researchers to compare hospital emergency room visits in Montreal and Boston and estimate the impact of the change. Just looking at US hospitals wouldn’t be enough, since there is a lot of year-to-year variation in the severity of the current flu strains, and just comparing the US to Canada wouldn’t work, since there are a whole lot of differences between the countries (starting with a national health insurance system). But looking at how the US:Canada difference changed from before 2006 to after 2006 gives a reasonable estimate of the effect of vaccination.
Looking at over 100,000 emergency-room visits for flu-like illness, researchers found a 34% decrease in risk for the 2-4 year age group affected by the change in guidelines. This wasn’t just for kids who were actually vaccinated — it also includes the reduction in risk from having your playmates vaccinated. There was a smaller reduction in risk, 10-20%, for older children — either because the additional reminders made them more likely to get vaccinated, or because they were less likely to catch flu from younger siblings.
Natural experiments get used a lot in economics. In medicine, we tend to prefer real experiments, but sometimes these are impractical or unethical, and natural experiments are the best we can do.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
Thomas, do you know of any other interesting ‘natural’ experiments (first time I have heard this term) and do you know of any New Zealand ones?
13 years ago
There’s an important one in air pollution research: a strike closed a steel mill in Steubenville, Ohio, that was the main source of air pollution in the surrounding area. During the 13-month strike, cardiovascular deaths dropped, and then went back up again when the mill re-opened.
A NZ example is the barriers to stop people jumping off the Grafton Bridge. These were removed for five years (1997-2002) and then replaced with improved barriers. With the old barriers the suicide rate by jumping off the bridge was about 1/year, increasing to 3/year when the barriers were removed, and dropping to zero with the new barriers installed.
13 years ago
Fascinating – thanks!
13 years ago