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

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

In January 2021, the elderly and frontline workers, who were at high risk of COVID-19 infection, were prioritized for SARS-CoV-2 vaccination. However, gradually it was extended to younger age groups up to 12 years old. Although immunization coverage increased rapidly, it slowed considerably at the end of the summer.

To study: Assessment of COVID-19 booster vaccination strategies in a partially vaccinated population: a modeling study. Image Credit: Romantic Studio / Shutterstock

The need for booster vaccines

The emergence of SARS-CoV-2 variants is due to mutations. Scientists classified the variants into variants of concern (VOC) and variants of interest (VOI). In February 2021, SARS-CoV-2 Alpha VOC became dominant in mainland 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 escape the immune response induced by vaccines or natural infection. Numerous studies have reported that the efficacy of the available COVID-19 vaccines is lower in the case of the Delta strain compared to other variants of SARS-CoV-2. Previous studies have also shown a decrease in immune protection, caused by vaccines or natural infection, over time. For these reasons, many countries have recommended COVID-19 booster shots for people who have already been vaccinated.

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

Efficiency on hospitalizations of vaccination strategies varying in the allocation of 200,000 daily doses between the primary vaccination and booster shots, over the period from September 1, 2021 to March 1, 2022, compared to a reference scenario in which any vaccination is discontinued on September 1, 2021. (A, C, E) The vaccine's effectiveness only decreased for people 65 years of age and 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 immunization and 50,000 daily booster doses until 90% coverage of a target population is achieved, then every 200,000 doses daily are allocated to the other target population.
Efficiency on hospitalizations of vaccination strategies varying in the allocation of 200,000 daily doses between the primary vaccination and booster shots, over the period from September 1, 2021 to March 1, 2022, compared to a reference scenario in which any vaccination is discontinued on September 1, 2021. (A, C, E) The vaccine’s effectiveness only decreased for people 65 years of age and 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 immunization and 50,000 daily booster doses until 90% coverage of a target population is achieved, then every 200,000 doses daily are allocated to the other target population.

Efficiency

In Europe, the number of COVID-19 cases has started to rise again. In this scenario, along with constraints in the global vaccine supply, researchers believe it is extremely important to assess the effectiveness of booster vaccination campaigns. The researchers used a deterministic, age-structured compartment model for this assessment. In this model, they adjusted the data associated with hospitalizations and deaths. The authors validated the seroprevalence data in France to determine the effect of primary vaccination and booster strategies on morbidity and mortality.

This study is available on the medRxiv* preprint server. In this study, scientists made two hypotheses, that is, decreased immunity and increased transmission of the virus in winter.

The researchers compared the vaccination strategies according to the prioritization between the primary vaccination and the booster vaccination. This study found that the most beneficial strategy was to dispense all available doses of COVID-19 vaccines primarily for primary vaccinations and the remaining doses for boosters. This result also underlines the fact that a higher immunity of the population plays a greater 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 were to be decreased in all age groups, the most effective approach to reduce hospitalizations and deaths from COVID-19 would be to provide booster vaccines for the age group 30-49 years. . In another scenario, where if the efficacy of the vaccine were to be reduced for people 65 years of age or older, a different booster vaccination could lead to varying results, in terms of hospitalization and mortality. Therefore, this study suggested two mechanisms on the basis of which policymakers could decide which group to target for booster vaccination.

The researchers suggested that the decision should be made on the basis of immunizing vulnerable groups who are prone to severe infection and those at high risk of transmission. The latter group will help reduce the transmission of infection and therefore protect the population from further 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 speculated that a recovered individual would no longer be reinfected. Although some studies have reported on the breakthrough of the infection, little evidence is available on the risk of re-infection after recovering from COVID-19.

Conclusion

The model presented in this study made it possible to determine the optimal vaccination strategy linked to the primary vaccination and booster approach. Although this study drew a conclusion based on data from mainland France, the authors believe the observations would hold for other countries as well. The emergence of the Omicron variant has shown the importance of vaccine coverage worldwide. Rapid global vaccination will help contain the COVID-19 pandemic sooner.

*Important Notice

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


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Sara H. Byrd