Study: Effect of population mixing between vaccinated and non-vaccinated subpopulations on the dynamics of infectious diseases: effects on SARS-CoV-2 transmission. Image credit: GoodStudio / Shutterstock
Record
During the severe Coronavirus 2 (SARS-CoV-2) pandemic of acute respiratory syndrome, the rate at which COVID-19 vaccines were developed was impressive. Although the unfair worldwide distribution of the SARS-CoV-2 vaccine and the emergence of viral variants that block the immune system posed a threat to the effectiveness of the vaccine, COVID-19 vaccines have saved many lives. The sentiment against COVID-19 vaccine fueled in part by coordinated misinformation campaigns has led to low vaccine absorption in several countries, leading to adverse economic and health effects. Although the option to refuse vaccination is sometimes framed as the individual’s freedom to choose, such arguments overlook the potential disadvantages for the wider community resulting from low vaccine intake. Non-vaccination is predicted to increase disease transmission among unvaccinated subpopulations. However, since infectious diseases are contagious, non-vaccination also increases the risk for vaccinated groups when vaccines provide only partial protection. In addition, because SARS-CoV-2 has an airborne transmission characteristic, short-range physical mixing of vaccinated individuals with vaccinated and unvaccinated cohorts is not required for disease transmission between groups.
About the study
The aim of the present study was to evaluate how the mixing of unvaccinated and vaccinated individuals with COVID-19 affected the risk of SARS-CoV-2 infection among vaccinated individuals. The researchers constructed a simple susceptible-infectious-recovered compartmental model of COVID-19 with two correlated subpopulations: vaccinated and unvaccinated individuals. In order to better understand the consequences of the interaction between these two populations, the researchers reproduced the interaction between vaccinated and non-vaccinated subpopulations in a substantially vaccinated community. The team established a variety of mixing patterns between vaccinated and unvaccinated cohorts, ranging from random mixing to full assimilation (such as mixing with similar ones), where humans interact only with those who had the same vaccination status. The researchers investigated the dynamics of an epidemic within each subgroup and throughout the population. They compared the contribution of the subpopulation with the size of the epidemic and the risk assessments. They then analyzed the effect of mixing unvaccinated and vaccinated individuals on the predicted disease dynamics.
Results
The results of the study showed that despite its simplicity, the present model provided a graphical representation of the hypothesis that even with highly effective COVID-19 vaccines and high vaccination coverage, a significant proportion of new cases will occur in vaccinated individuals. This showed that percentages, not absolute numbers, were the logical measure of the impact of vaccination. However, the researchers found that the degree to which individuals were differently involved with individuals with a similar vaccination status significantly influenced the dynamics and risk of disease in individuals who chose to be vaccinated.
Effect of mixing between vaccinated and non-vaccinated subpopulations on risk contribution and final epidemic size for (A) different breeding numbers and (B) vaccine efficacy. Both panels show the impact of an increase in outbreak-like mix between the vaccinated subpopulation and the contact-adapted contribution to the risk of infection in vaccinated individuals from unvaccinated individuals (ψ). As similar mixing with similar (h) increases, the rate of infection among vaccinated individuals decreases, but ψ increases. This relationship is shown in a range of (A) initial reproduction numbers and (B) vaccine efficacy. These effects are more pronounced at lower reproduction rates and weaken as the vaccines become less effective. We used a baseline estimate of 6 for the number of reproductions in the sensitivity analysis on vaccine efficacy and a baseline estimate of the efficacy of the vaccine 0.8 in the susceptibility assay for R. Random mixing of vaccinated individuals with unvaccinated individuals reduced the incidence of SARS-CoV-2 infestation between the last cohort, acting as a virus transmission buffer. In addition, the risk of infection was significantly higher among unvaccinated individuals than among those vaccinated in all mixed models. Unvaccinated participants made a disproportionate contribution to the risk of infection after adjusting the number of contacts. The authors observed that unvaccinated individuals became infected at a higher rate than predicted based on contact numbers alone. COVID-19 infection rates among vaccinated individuals decreased from 15% to 10% when similar mixing expanded and increased from 62 to 79% among unvaccinated individuals. However, the contact-controlled contribution to the risk of vaccinated individuals resulting from interaction with unvaccinated individuals increased. Given that this excessive contribution to risk could not be tackled with a high degree of similarity, the notion that vaccination was a personal choice and supports strong public action people. The researchers also said that regulatory and legal measures to control practices and behaviors that put the public at risk have spread to communicable diseases in the past, such as banning smoking in public places. The researchers found that when vaccination efficacy was low, mixing similar to it was less protective in the context of the immune leak that occurred with the recent SARS-CoV-2 Omicron variant. This finding underscores the dynamic nature of the pandemic and the need for policy adaptation as the nature of the disease and the protective effects of vaccines change.
conclusions
Collectively, the present study demonstrated that although the risk of non-vaccination during a severe pandemic falls mainly on unvaccinated individuals, their decisions affect the likelihood of viral infection among those vaccinated in a manner disproportionate to the number of unvaccinated individuals in the community. The authors report that unvaccinated individuals face a risk that can not be considered self-respecting. In addition, equality and equity concerns for those who choose to be vaccinated and those who choose not to be vaccinated must be taken into account in designing the vaccination policy. Given the wide range of susceptibility assays, current findings can be used in future evaluations when new variants of SARS-CoV-2 emerge and new vaccine formulations are available, as it illustrates how long the vaccine provides protection.
