The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has claimed millions of lives worldwide. However, with the availability of vaccines, there was hope that this unprecedented epidemic would come to an end.
In the early stages of the COVID-19 pandemic, an age-regulated vaccination model was used, which kept vaccination coverage below average for some segments of the population, and thus affected them more during the fourth or fifth wave of the epidemic.
Therefore, even when more than half of the population of many Western countries is fully vaccinated, there is a need to adjust the vaccination strategy and develop a strategy that prioritizes achieving herd immunity with limited vaccine supplies.
This vaccination strategy is based on reducing the base reproduction number indicated by R0 It will work by providing greater protection for those age groups who, for a certain social contact pattern, are more susceptible to SARS-CoV-2 infection.
Modification of vaccination strategy to develop herd immunity against SARS-CoV-2
Vaccination campaigns are boring and put a tremendous strain on the health care and economic systems of even the most developed countries. Moreover, the vaccines gave short-lived immunity (weakened immunity), which means that their effect began to wear off after 12-18 months, leaving the vaccinated individuals vulnerable to infection again.
Researchers from the United States used a simple SIRV (Appendix) epidemic model and focused on developing a vaccination strategy to develop herd immunity. This study mainly focused on the basic reproductive number R .0 For populations with only vaccine-susceptible individuals, the chosen model ignored some relevant aspects of COVID-19 dynamics, such as the presence of a latent period and different levels of disease severity.
Herd immunity may not be achieved at all in many countries due to limited availability of vaccines or the emergence of new types of virus that can alter the herd immunity threshold itself. “
The vaccination strategy can be designed with various goals, including reducing the number of cases, reducing mortality, reducing severe disease leading to hospitalization, etc.
In this study, the researchers aimed to find a minimum vaccination rate for each age group that would guarantee a basic reproductive number R .0 It was less than 1, and thus the dynamics stabilized at disease-free equilibrium. The disease parameter values chosen in the study simulate approximately the infectious period of COVID-19 (about 1 week) and current estimates of the likely duration of immunity (approximately 1 year).
The researchers used a deterministic epidemiological model with continuous vaccination of individuals of 3 age groups: young, adult and elderly. Within each age group, individuals were again classified as susceptible, contagious, homozygous, and vaccinated, based on their disease status. It was assumed that there would be a loss of immunity in recovered and vaccinated individuals at different rates and that the probability of the vaccine being successful in protecting against infection depended on age.
Examples of development of susceptible individuals during an epidemic before (1) without vaccination with initial status (si (0), yi (0), ri (0)) = (0.9999, 0.0001, 0) for i = 1, 2, 3 Parameters: γ1 = 1, γ2 = 1, γ3 = 0.9, 1 = 1/40, δ2 = 1/52, δ3 = 1/40. For each country, it was measured so that R0 = 2.5 for the corresponding data set in the absence of vaccinated individuals.
The researchers had hoped that the specific vaccination rate for each age group would result in a net vaccination rate lower than the critical vaccination rate that brings R0 to 1 when considering homogeneous intermediate connection patterns. They also studied the effect of population heterogeneity on herd immunity by dividing the population into six age groups with appropriate heterogeneous contacts for different social activities to demonstrate that herd immunity can be reached at around 43% instead of the traditional 60%.
The authors of this study published in medRxiv* The prepress server also investigated the amount of vaccines required by any given country, year after year. They show the percentage of the population that should be vaccinated in the first year of an epidemic versus the percentage that should be vaccinated a few years later once the system reaches equilibrium.
Vaccination campaigns must continue despite the COVID-19 pandemic
This is a groundbreaking study, the results of which encourage ongoing vaccination campaigns by establishing vaccination as the only possible way to end this ongoing epidemic along with giving insight into the role of communication patterns in spreading disease and defining an optimal vaccination strategy.
The results of this study also clearly demonstrate that post-vaccination reduction can be achieved with R.0 Minimization strategy by decreasing the overall availability of vaccines per unit time. The observations also highlight the need to consider demographic and social aspects to improve vaccination distribution.
“R .”0 It can play a critical role in determining the minimum vaccination coverage to prevent new epidemic invasions through access to so-called herd immunity.”
medRxiv publishes preliminary scientific reports that have not been peer-reviewed and therefore should not be considered conclusive, guide clinical practice/health-related behavior, or be treated as established information.