Effectiveness of COVID-19 booster vaccination strategies in partially vaccinated populations

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Effectiveness of COVID-19 booster vaccination strategies in partially vaccinated populations


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In France, four coronavirus vaccines 2019 (COVID-19), developed by Pfizer / BioNTech, Moderna, AstraZeneca and Janssen-Cilag, have received an emergency use authorization (US) and a rapid vaccination program to protect its citizens from infection. The COVID-19 pandemic has been caused by Coronavirus 2 of Severe Acute Respiratory Syndrome (SARS-CoV-2) which has been characterized by being highly transmissible, virulent, and with a high mortality rate.

In January 2021, seniors and front-line workers, who were at high risk for COVID-19 infection, were prioritized for SARS-CoV-2 vaccination. However, it gradually spread to younger age groups up to the age of 12. Although vaccination coverage increased rapidly, it decreased significantly in late summer.

Study: Evaluation of COVID-19 reinforcement vaccination strategies in a partially vaccinated population: a modeling study. Image credit: Studio Romantic / Shutterstock

Need for booster vaccines

The appearance of SARS-CoV-2 variants occurred due to mutations. Scientists have classified the variants as variants of concern (VOC) and variants of interest (VOI). In February 2021, SARS-CoV-2 Alpha VOC became dominant in metropolitan France, which was replaced by Delta VOC. Currently, the Delta variant has become the dominant circulating strain and has been characterized as more virulent and transmissible than the original SARS-CoV-2 strain. It can also evade the immune response induced by vaccines or natural infection. Many studies have reported that the efficacy of available COVID-19 vaccines has been lower in the case of the Delta strain compared to other SARS-CoV-2 variants. Previous studies have also shown a decrease in immune protection, caused by vaccines or natural infections, over time. For these reasons, many countries have recommended COVID-19 booster vaccinations for people who have already been vaccinated.

The French government had initially recommended a booster dose of COVID-19 for people over the age of 65, as well as for other vulnerable groups, six months after completing the two-dose vaccination regimen. More recently, however, eligibility for the booster dose has been extended to all adults.

Effectiveness in hospitalizations of vaccination strategies that vary in the allocation of 200,000 daily doses between primary vaccination and booster injections, during the period between September 1, 2021 and March 1, 2022, in comparison with a baseline scenario in which all vaccination stops on September 1, 2021. (A, C, E) Vaccine efficacy decreased only for people 65 years of age or older.  (B, D, F) Vaccine efficacy decreased for all age groups.  (A, B) New daily hospitalizations, (C, D) proportion of hospitalizations avoided, (E, F) proportion of hospitalizations avoided by age group.  A prioritization strategy of (150,000 / 50,000) means 150,000 daily doses for primary vaccination and 50,000 daily booster doses until coverage of 90% of a target population is achieved, and then all 200,000 daily doses are they assign to the other target population.
Effectiveness in hospitalizations of vaccination strategies that vary in the allocation of 200,000 daily doses between primary vaccination and booster injections, during the period between September 1, 2021 and March 1, 2022, in comparison with a baseline scenario in which all vaccination stops on September 1, 2021. (A, C, E) Vaccine efficacy decreased only for people 65 years of age or older. (B, D, F) Vaccine efficacy decreased for all age groups. (A, B) New daily hospitalizations, (C, D) proportion of hospitalizations avoided, (E, F) proportion of hospitalizations avoided by age group. A prioritization strategy of (150,000 / 50,000) means 150,000 daily doses for primary vaccination and 50,000 daily booster doses until coverage of 90% of a target population is achieved, and then all 200,000 daily doses are they assign to the other target population.

Effectiveness

In Europe, the number of COVID-19 cases has started to rise again. In this scenario, along with the limitations in the global supply of vaccines, researchers believe that it is extremely important to evaluate the effectiveness of booster vaccination campaigns. Researchers have used a deterministic, age-structured compartmentalized model for this assessment. In this model, they adjusted data that were associated with hospital admission and deaths. The authors validated seroprevalence data in France to determine the effect of primary and reinforcement vaccination strategies on morbidity and mortality.

This study is available at medRxiv* Prepress server. In this study, scientists made two assumptions, namely, decreased immunity and improved transmission of the virus in winter.

The researchers compared vaccination strategies based on prioritization between primary vaccination and booster vaccination. This study revealed that the most beneficial strategy was to distribute all available doses of COVID-19 vaccines predominantly for primary vaccines and the remaining doses for booster vaccines. This result also emphasizes that the immunity of the higher population plays a more important role in the protection of vulnerable groups than in the protection of individuals.

The current study reported that in the scenario where if the effectiveness of vaccines was reduced among all age groups, the most effective approach to reduce hospitalization and death due to COVID-19 would be to provide booster vaccines for the 30-49 age group. . In another scenario, where the effectiveness of the vaccine for people aged 65 and over is reduced, a different booster vaccination could lead to different outcomes, in terms of hospitalization and mortality. Therefore, this study suggested two mechanisms based on which health policy makers could decide which group to target for booster vaccination.

The researchers suggested that the decision should be made on the basis of immunization of vulnerable groups that are prone to serious infections and those at high risk of transmission. The latter group will help reduce the transmission of the infection and therefore protect the population from new infections. Therefore, an optimal booster vaccination strategy would be based on protecting the vulnerable group as well as the general population by inhibiting viral transmission. The researchers assumed that a recovered individual would not become infected again. Although some studies have reported innovative infection, not much evidence is available on the risk of reinfection after recovery from COVID-19.

Conclusion

The model presented in this study helped determine the optimal vaccination strategy related to the primary and booster vaccination approach. Although this study has drawn a conclusion based on data from metropolitan France, the authors believe that the observations would also be valid for other countries. The advent of the Omicron variant has indicated the importance of vaccination coverage worldwide. Rapid global vaccination will help contain the COVID-19 pandemic sooner rather than later.

* Important news

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guided by clinical practice / health-related behaviors, or treated as established information.

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