In a recent study posted to the medRxiv* preprint server, researchers explored the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) booster vaccination necessity among various age groups in Israel.
In this study, the researchers also explored the effect regarding preventing disease outcomes, and the impact of time on initiation of boosters via a mathematically calibrated model based on coronavirus disease 2019 (COVID-19) data from Israel.
Israel was one of the first countries that initiated mass COVID-19 vaccination campaigns, and they were also among the countries that first experienced significant breakthroughs due to the waning vaccine-induced immunity that led to a resurgence of the SARS-CoV-2 epidemic.
To contain the SARS-CoV-2 breakthrough rate, Israel initiated a booster vaccination campaign, which was demonstrated to be efficient in curtailing the resurgence of the Delta variant by only implementing minimum non-pharmaceutical interventions without a lockdown. However, there is uncertainty regarding the requirement of boosters among various age groups and the appropriate time of initiation.
In the present study, the researchers constructed a discrete-time age-of-infection age-stratified transmission model to accurately determine the individual to population-level data on SARS-CoV-2 Delta surge dynamics in Israel from July 1-November 25, 2021.
The researchers compared the impact of alternative vaccination policies such as vaccinating different age groups and early and extensive booster rollout. Additionally, the model explored direct and indirect protection from different outcomes of COVID-19 imparted by the booster vaccination.
The participants’ data on age, location of residence, dates of first, second, third COVID-19 vaccination, polymerase chain reaction (PCR) test, COVID-19-associated hospitalization, and death were collected from the Israel Ministry of Health database. COVID-19 disease severity was defined based on the National Institutes of Health COVID-19 treatment guidelines.
In the mathematical model, the SARS-CoV-2 infection process was modeled using a social contact matrix comprised of social settings such as household, community, work, and school, which determines the number of average daily interactions.
The researchers assumed that the waning effect of vaccine protection from severe SARS-CoV-2 outcomes is equal to the waning vaccine protection from SARS-CoV-2 infection, and this was validated using the relative estimates. Further, they assumed the waning protection of booster from SARS-CoV-2 infection and severe disease was half that associated with the second COVID-19 vaccination.
The results indicated that without non-pharmaceutical interventions or booster vaccinations, the Delta wave had the potential to cause an increase of approximately 385% in detected infections, 763% in severe disease, and 880% in deaths. However, booster vaccination of nearly 40% of the population was associated with a 70% reduction in the number of SARS-CoV-2 cases.
There was a significant reduction in the number of cases, severe diseases, and mortality when the booster vaccination was restricted to only 40 years or older and 60 years or older age groups. However, the restriction of booster vaccines to certain age groups was less effective in all aspects compared to booster vaccination of more population segments.
The model demonstrated that at least two weeks’ earlier initiation of booster vaccination might have resulted in an overall decrease in the number of infections to about a factor of three than the actual booster vaccine rollout.
In Israel, those above 60 years of age had a higher number of social contacts during the Delta outbreak than the previous SARS-CoV-2 waves, according to the POLYMOD contact matrix in this age group. The 60 years or older age group was associated with a basic reproduction number of 2% in the POLYMOD contact matrix adapted to Israel whereas the basic reproduction number contributed by the 60 years or older age group varied between 10-11% in the time-varying social contact matrix used in the simulation.
The vaccine uptake rate was lower in the younger age groups, where the uptake is crucial in determining the impact of booster vaccines.
The study demonstrated the impact of COVID-19 booster vaccination from the individual to the population level in Israel using a mathematical calibrated model. The booster rollout was efficient in curtailing the resurgence of the COVID-19 associated with the waning immunity from the vaccines.
The findings highlighted the need of vaccinating younger populations who are not at a higher risk of severe SARS-CoV-2 disease but play a significant role in COVID-19 transmission.
Further, the study indicated that the efficacy of the booster vaccine mainly depends upon the timing of its initiation in controlling the epidemic and hence warrants the importance of a rapid response to reduce the disease burden through the implementation of a booster vaccination.
Overall, in the light of newer SARS-CoV-2 variants such as Omicron, it is of prime importance to adopt new strategies that will help reduce the disease burden of COVID-19.
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.