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How does the rate of vaccine uptake affect the course of diseases like COVID-19 or the measles? And what makes people more or less willing to be vaccinated?

Vaccine hesitancy—delaying or refusing available vaccines—can be influenced by factors like politics, the media, trust in healthcare and personal beliefs. It often aligns with age, income, background and political views.

People with similar views tend to group together, a pattern called homophily. This can lead to clusters of unvaccinated individuals, reducing vaccine effectiveness in communities.

ÓÈÎïÊÓƵ (SFU) mathematics professor Caroline Colijn and health sciences professor Kiffer Card both study public health trends. Colijn, the Canada 150 Research Chair in Mathematics for Evolution, Infection and Public Health, studies the ecology and evolution of infections using mathematical tools. Card is the inaugural Blanche and Charlie Beckerman Public Health Innovation Scholar and researches the ecological, political, and social determinants of health.

Colijn and Card worked with SFU postdoctoral fellow to investigate the impact of vaccine homophily on COVID-19 transmission within and between vaccination groups that had both high and low vaccine efficacy.

Their study, , surveyed close to 1200 people in B.C. in February and March 2022 about their vaccination status, size of contact groups and COVID-19 prevention measures. It also asked whether they had ever had COVID-19.

The authors examined vaccine homophily and related it to participants’ level of vaccination. They observed a high rate of infection among unvaccinated individuals under conditions of high vaccine efficacy, as well as an elevated infection rate from unvaccinated to sub-optimally vaccinated individuals under conditions of low vaccine efficacy.

We spoke to professors Colijn and Card about their research.

Can you describe the general findings of the study?

Our study found that vaccinated individuals tended to associate with others who were also vaccinated (vaccine homophily), had smaller contact networks, and followed public health guidelines more closely. Unvaccinated individuals had larger, more diverse social networks and were more likely to report having a prior COVID-19 infection.

Did you use modelling to determine how widespread COVID-19 vaccination was? Were the COVID-19 vaccines effective in curbing the spread of the illness?

Yes, modelling incorporated survey data and vaccination coverage in B.C. to simulate transmission dynamics. Vaccines were effective in curbing transmission, especially when uptake was high and homophily reduced mixing between vaccination groups. against severe illness, lower the risk of hospitalization and fatality, reduce the number of sick days, and protect against the risk of long COVID.

Regarding the , what can we learn from COVID-19 about disease transmission and community level immunity?

The study highlights how clusters of unvaccinated individuals undermine community immunity, enabling disease spread even when vaccines are effective. This has direct implications for highly contagious diseases like measles. Last spring, that if Canada’s measles vaccination rate fell below 85 per cent, we could see 100s of cases. 

Based on your research and expertise, what types of initiatives are required to ensure at-risk groups in B.C. are able to access the healthcare information and interventions they need?

Community-based, culturally sensitive outreach is needed to address vaccine hesitancy and misinformation. Tailored interventions that build trust and directly engage unvaccinated populations are essential for equitable health protection.

Is there anything further you would like to mention?

We are grateful for the leading contributions of postdoctoral researcher Elisha Are.

For more visit:

• The Pacific Institute on Pathogens, Pandemics and Society
  
• The
• The Mathematics, Genomic and Prediction Infectious and Evolution (MAGPIE) Lab