The Impact of Seasonal Birth-Rate Fluctuations on Measles, Other Vaccine-Preventable Diseases
Audrey Dorélien, PhD, is a 2012-2014 Robert Wood Johnson Foundation (RWJF) Health & Society Scholar studying demography, infectious diseases, and maternal and child health.
Reoccurring outbreaks of measles and other vaccine-preventable diseases are a major killer of children, particularly in sub-Saharan Africa. In 2012, more than 226,000 cases of measles were reported worldwide, with a little less than half of those in Africa. For the World Health Organization to meet its global measles eradication goal and implement more effective supplemental vaccination programs, public health officials will need a better understanding of the mechanism driving seasonal and episodic outbreaks.
Infectious disease ecologists have demonstrated the importance of human demography, and in particular the influence of the birth rate on the dynamics of acute childhood immunizing (ACI) diseases. For instance in London, in the few years prior to 1950, the city experienced annual measles epidemics, but the dynamics changed to biennial epidemics as a result of a decline in the birth rate between 1950 and 1968. How can the birth rate influence disease outbreaks? An outbreak can only occur when the fraction of the susceptible population exceeds a critical threshold. In the case of ACI disease, the majority of the susceptible population are young children; therefore the birth rate influences the rate at which the pool of susceptibles is replenished.
As a demographer—someone interested in human population dynamics—I wondered whether seasonal fluctuations in human birth rates could influence the epidemics of such acute immunizing diseases as measles, especially in places with both large birth rates and large seasonal fluctuations in the birth rates. I was also interested in whether there might be important interactions with known factors, such as seasonal changes in transmission. From a policy perspective, seasonal fluctuations in birth rates are predictable, so if indeed they play a role, they should be included in models of ACI diseases.
To get answers, I modified a well-known epidemiological model for measles to allow birth and transmission rates to fluctuate seasonally. I then simulated the effect of varying the birth rate and amplitude (the degree of seasonal fluctuations) based on ranges of measles incidence in sub-Saharan Africa. I found that varying birth seasonality had an effect both on the timing of the epidemic and its magnitude, and seasonality could also influence the periodicity of the epidemics. In addition, I found important interactions between seasonality in birth rates and contact rates—the rates at which people came into contact with the measles virus, whether they contracted the disease or not.
Including seasonal fluctuations in birth rates into epidemiological models may help improve our understanding of the spread of the disease and therefore allow us to better characterize and check measles epidemics in settings where outbreaks still occur regularly. It could also help improve the timing of supplemental vaccination programs, so that they may take into account the large inflow of susceptible infants at certain times of the year.
I am very excited that other researchers are starting to include birth seasonality in their models in both developing and developed countries. A recent paper by colleagues at the University of Michigan, Micaela Martinez-Bakker and Kevin Baker, echoes my findings. Using historical data from the United States, they found that the geographical variation in the timing and amplitude of birth seasonality had the potential to influence the timing and magnitude of measles epidemics.
Learn more and read my paper here.
2] Earn et al 2000 DOI: 10.1126/science.287.5453.667 [return]