I spent last week in Geneva at a working group meeting to develop recommendations for measles and rubella vaccination policy. The recommendations themselves won't be formalized until a November meeting, where they will be presented to a panel that will review the support for each and decide on their adoption. The working group, which meets frequently through the year to evaluate current progress towards measles and rubella control goals, was tasked with evaluating changes to current recommendations in light of three important trends: the resurgence of measles outbreaks in places that had experienced long periods of measles absence, the increasing age of measles cases in these outbreaks, and the increasing occurrence of nosocomial outbreaks.
The former two are not surprising given the expected dynamics of measles when prevalence declines due to successful control (see here). Lower prevalence increases the likelihood of stochastic extinction -- at which point a regime of both natural and vaccine sources of immunity is replaced by a vaccine only regime which will necessarily allow susceptibles to accumulate faster. If vaccination remains below the levels necessary to prevent this accumulation, then outbreaks are likely to happen when and if measles is re-introduced. And as prevalence declines, and these periods measles absence progress, those who have not been vaccinated or infected in the past will get older -- and when measles returns, the pool of susceptibles will include these older individuals leading to an increase in the mean age of infection. Unfortunately, while we know that these patterns should occur in theory, it is difficult to pinpoint exactly whether or not the frequency of outbreaks or shift in age that we're seeing now is due only to these processes or any other changes in measles epidemiology. Further, even if everything is proceeding exactly as theory suggests, the experience of each country and its vaccination program is really a one-off experiment -- so developing comprehensive recommendations for how health systems should respond to these changes that will work in all countries is no simple task.
Though its often easy to criticize those to write policy for not considering all the nuances and caveats that academics often like to obsess over, there is a premium put on simplicity. Simple and concise messages are more likely to be understood and more likely to be acted upon. Often as scientists, we shy away from making such simplistic statements in the hopes that, given a little more time, we'll understand things a bit better, and be able to provide a slightly better answer. However, to do so leaves the rest of the world in the dark. As ever, my excursions to the policy realm impress upon me that while we often revere the scientists that make grand new discoveries, the bulk of the progress in the world is made through incremental steps.