The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has claimed millions of lives worldwide. However, as vaccines became available, there was hope that this unprecedented pandemic would end.
In the early stages of the COVID-19 pandemic, an age-structured vaccination model was used, which kept vaccination coverage lower than average for some sections of the population, therefore impacting them the most during the fourth or fifth wave of the pandemic.
So even when more than half of the population of several western countries have become fully vaccinated, there is a need to modify the vaccination strategy and develop one that prioritizes achieving herd immunity under a limited supply of vaccines.
This vaccination strategy based on minimizing the basic reproduction number denoted by R0 will work by giving greater protection to those age groups that, for a given social contact pattern, are more vulnerable to SARS-CoV-2 infection.
Altering the vaccination strategy for developing herd immunity to SARS-CoV-2
Vaccination drives are tedious and impose huge pressure on the healthcare and economic systems of even the most developed nations. Moreover, vaccines gave short-lived immunity (waning immunity), implying their effect started vanishing after 12-18 months, making vaccinated individuals susceptible to infection again.
Researchers from the US used a simple SIRV(accinated) epidemic model and focused on developing a vaccination strategy for developing herd immunity. This study mainly focused on the basic reproduction number R0 for populations with only susceptible and vaccinated individuals, and the model chosen ignored some relevant aspects of the COVID-19 dynamics, such as the existence of a latent period and various levels of disease severity.
“Herd immunity may not be achieved at all in several countries due to the limited availability of vaccines or the appearance of new variants of the virus that could change the herd immunity threshold itself. “
A vaccination strategy can be designed with different goals, including minimizing the number of cases, minimizing deaths, minimizing severe disease leading to hospitalization, etc.
In this study, researchers aimed at finding the minimum vaccination rate for each age group that guaranteed the basic reproduction number R0 was less than 1, thus stabilizing the dynamics at the disease-free equilibrium. The values of the disease parameters chosen in the study roughly mimic the COVID-19 infectious period (1 week) and the current estimates of the potential duration of the immunity (about 1 year).
The researchers used a deterministic epidemic model with continuous vaccination in individuals of 3 age groups: youngsters, adults, and the elderly. Within each age group, individuals were again classified as susceptible, infectious, recovered, 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 successfully protecting against infection is age-dependent.
Examples of evolution of susceptible individuals during an epidemic given by (1) without vaccination with initial condition (si(0), yi(0), ri(0)) = (0.9999, 0.0001, 0) for i = 1, 2, 3. Parameters: γ1 = 1, γ2 = 1, γ3 = 0.9, δ1 = 1/40, δ2 = 1/52, and δ3 = 1/40. For each country, β is scaled such that R0 = 2.5 for the corresponding data set in the absence of vaccinated individuals.
The researchers hoped that a specific vaccination rate for each age group would result in a net vaccination rate smaller than the critical vaccination rate that brings R0 to 1 when considering homogeneous average contact patterns. They also studied the influence of population heterogeneity on herd immunity by dividing the population into six age groups with heterogeneous contacts fitted to various social activities to demonstrate that herd immunity can be reached at about 43% instead of the traditional 60%.
The authors of this study published in the medRxiv* pre-print server also investigated the quantity of vaccines required by any given country, year by year. They demonstrated the percentage of the population that must be vaccinated in the first year of an epidemic vis-à-vis the percentage that must be vaccinated after a few years once the system reaches equilibrium.
Vaccination drives should continue despite COVID-19 infodemic
This is a path-breaking study, and its findings encourage continuing vaccination drives by establishing vaccination as the only way to possibly end this ongoing pandemic besides giving a clear insight into the role of contact patterns in spreading disease and establishing the definition of an optimal vaccination strategy.
The results of this study also clearly establish that vaccine reduction could be achieved following the R0 minimization strategy by decreasing the total availability of vaccines per unit of time. The observations also highlight the need to consider demographic and social aspects to improve vaccination distribution.
“R0 can play a critical role in defining the minimum vaccination coverage for preventing new epidemic invasions by reaching the so-called herd immunity.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.