The importance of increasing primary vaccinations against COVID-19 in Europe

Abstract

In the European Union, mass vaccination against COVID-19 staved off the strict restrictions that had characterized early epidemic response. Now, vaccination campaigns are focusing on booster doses, and primary vaccinations have all but halted. Still, 52 million European adults are unvaccinated. We investigated if reaching the still unvaccinated population in future vaccination campaigns would substantially decrease the current burden of COVID-19, which is substantial. We focused on vaccination homophily, whereby those who are unvaccinated are mostly in contact with other unvaccinated, making COVID-19 circulation easier. We quantified vaccination homophily and estimated its impact on COVID-19 circulation. We used an online survey of 1,055,286 people from 22 European countries during early 2022. We computed vaccination homophily as the association between reported vaccination status and perceived vaccination uptake among one's own social contacts, using a case-referent design and a hierarchical logistic model. We used this information in an analysis of the COVID-19 reproduction ratio to determine the impact of vaccine homophily in transmission. Vaccination homophily was present and strong everywhere: the average odds ratio of being vaccinated for a 10-percentage-point increase in coverage among contacts was 1.66 (95% CI=(1.60, 1.72)). Homophily was positively associated with the strictness of COVID-19-related restrictions in 2020 (Pearson = 0.49, P = .03). In the countries studied, 12%-to-18% of the reproduction ratio would be attributable to vaccine homophily. Reducing vaccination homophily may curb the reproduction ratio substantially even to the point of preventing recurrent epidemic waves. In addition to boosting those already vaccinated, increasing primary vaccination should remain a high priority in future vaccination campaigns, to reduce vaccination homophily: this combined strategy may decrease COVID-19 burden.

Keywords: COVID-19; Europe; Reproduction ratio; Survey; Vaccination campaign; Vaccination homophily.

Fig. 1

Vaccination homophily. (A) Probability of being vaccinated conditioned on gender, age and reported perceived coverage across the countries under study (see Supplemental File for computation and Supplemental File Fig. S3 for country-specific probabilities): in blue model estimates reporting posterior median and 95% credibility interval; in red empirical frequencies with binomial 95% confidence intervals. (B) and (C) Posterior estimates of vaccination homophily in terms of posterior vaccination odds ratio for 10% increase in perceived coverage for each country under study: we report median values in B,C and 95% credibility intervals in B. (D) Scatter plot of vaccination coverage (April 2022) vs vaccination homophily. Countries in blue joined the EU before 2004 (Western Europe), those in red in 2004 or after (Eastern Europe). Countries in gray are in the ECDC network but not members of the EU. (E) Scatter plot of Stringency Index (averaged over March–December 2020) and vaccination homophily. Color code is the same as (D).

Fig. 2

Impact of vaccination homophily on the reproduction ratio. (A) Percentage of the reproduction ratio attributable to vaccination homophily, assuming that vaccination is 50% effective in protecting from infection, contact matrices from the POLYMOD study and that homophily applies to ¼ of non-household contacts. (B) Percentage of the reproduction ratio attributable to vaccination homophily at varying vaccination efficacy.