Mandatory vaccinations may not be necessary to eradicate some diseases
University of Guelph
GUELPH — Mandatory vaccination policies may not be necessary to eradicate some infectious diseases, according to new research by University of Guelph mathematics professor Chris Bauch.
"Conventional wisdom and conventional theory tell us that when infection can potentially be spread to almost everyone in a community, such as with measles, a disease outbreak can never be contained using voluntary vaccination," said Bauch. "However, our work shows conventional wisdom may be wrong for diseases that are spread primarily through close contact, such as smallpox."
The new study by Bauch and co-author Ana Perisic appears in the Feb. 6 issue of the open-access journal PLoS Computational Biology.
(Here is a link to the study: http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000280 )
Previous studies have suggested that voluntary programs cannot be 100-per-cent effective due to the self-interested behaviour of individuals: when vaccine coverage is very high, the disease becomes rare and the individual incentive to vaccinate disappears.
But most mathematical models used in these studies assume that populations mix homogenously – in effect, that an individual is just as likely to be infected by a complete stranger as by a close friend or family member. But that is not how infections spread with diseases like smallpox or SARS, which are predominantly spread through close social contacts.
In this new study, Bauch and Perisic analyze "free-rider" effects under voluntary vaccination for vaccine-preventable diseases where disease transmission occurs in a social network. Individuals choose whether to vaccinate based on the risk of infection from their neighbours and any risks associated with the vaccine itself.
Neighbours of an infected person will vaccinate as soon as their neighbour’s symptoms appear, so when neighbourhood size is small, voluntary vaccination results in rapid containment of an outbreak. As neighbourhood size increases, a threshold is reached beyond which the infection can break through due to the decisions of neighbours who choose not to vaccinate.
"This approach injects greater realism into the transmission modelling of close contact infections and gives us a much more nuanced picture of how people's behaviour influences the effectiveness of voluntary versus mandatory vaccination policies," said Bauch.
"For those pathogens that are difficult to transmit, the conventional wisdom that free-rider effects will make eradication difficult under voluntary vaccination may be wrong."