Importance of vaccination: the Sick Santa model


A researcher in Japan has raised a seasonally timely question: what if Santa Claus had a communicable disease as he made his rounds (Furuse Y. Med J Aust 2019;211:523-524)?

Disease transmission for influenza and measles was simulated using a SEIR (susceptible, exposed, infectious and recovered) compartmental model. Assumptions were that contact with individual children would be transient (they should be in bed), and different levels of transmission efficiency (a “Santa factor” from 0.01 [low] to 1.00 [high]) were used.

For the influenza simulation, it was assumed that Santa made his rounds in the Northern hemisphere during a seasonal influenza outbreak. The population infection rate without Santa was 18.7%. In 100 simulations, there was a significant increase in the influenza rate (to 20.9%) only if the Santa factor was 1 (equivalent to a generic adult in the general population). There was no increased risk of influenza if Santa behaved like Santa and most children were good and stayed in their rooms.

For the measles simulation, Santa was introduced to a population of 10,000 people on day 1. In 70% of the simulations, Santa (or one infected adult) did not cause any disease transmission – provided that 95% of children and 90% of adults were vaccinated. In 30% of simulations, no more than 150 people were infected.

What was especially noteworthy was when the simulation used a vaccination rate of 85% for children and 90% for adults. In this scenario, in 64% of simulations there was no transmission, in 15% there was a small outbreak, and in 21% there was a large outbreak (>150 cases) with the lowest Santa factor. If the Santa factor was high, all simulations resulted in a large outbreak of measles.

The author concluded that disease transmission by Santa Claus has the potential to have a large impact on public health. Recommendations were that Santa should minimize contact time with children while distributing presents, wash his hands frequently and wear a mask.

Perhaps most importantly, the study serves to illustrate how even a small reduction in the measles vaccination rate (from 90% to 85% in children) can have a disproportionately large impact on herd immunity and population health.

Recommend to a Colleague Go back to home page