Exploring COVID-19 vaccination prioritization strategies for primary and booster doses

In a recent study posted to the medRxiv* preprint server, researchers evaluated the coronavirus disease 2019 (COVID-19) vaccine, prioritizing options for primary and booster doses.

Study: Evaluating primary and booster vaccination prioritization strategies for COVID-19 by age and high-contact employment status using data from contact surveys. Image Credit: M-Foto/Shutterstock
Study: Evaluating primary and booster vaccination prioritization strategies for COVID-19 by age and high-contact employment status using data from contact surveys. Image Credit: M-Foto/Shutterstock

Background

The debate surrounding severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine prioritization has centered on weighing the advantages of: (1) the protective action imparted by the vaccine among those with the highest likelihood of severe clinical outcomes and (2) the indirect protection conferred by immunizing those at highest infection risk and transmission of the virus. While individuals aged 65 and older had the highest risk of experiencing severe disease and death due to COVID-19, studies note that essential service workers and others with high contact rates may be at a greater risk of contracting and transmitting COVID-19. Unfortunately, relatively little data have been collected to comprehend the variation of contact rates and hazards among different demographic categories.

About the study

In the present study, researchers assessed vaccination priority techniques based on age and labor status.

The Berkeley Interpersonal Contact Survey (BICS) survey is an online questionnaire designed to capture the number and character of respondents' physical and verbal interactions over 24 hours. Respondents (egos) of the BICS survey were questioned about household members and their non-household connections, as well as up to three of their contacts from the previous day (alters).

Respondents of the BICS were divided into three categories. These were low contact (LC) adults (age 18-64) reporting no close work contacts, high contact (HC) adults (age 18-64) reporting close work contacts, as well as adults 65 or over.

Almost 34% of subjects aged between 18 and 64 had interpersonal connections at work and were classified as HC. Alters were classified into children between zero and 18 years, adults aged between 18 and 64 years, and people aged 65 years and above.

The team employed a deterministic and continuous-time compartmental model to simulate SARS-CoV-2 transmission dynamics and COVID-19 incidence and death. The model permits heterogeneous mixing among age and employment status groups, as indicated by the contact matrix produced from BICS survey results. When exposed to SARS-CoV-2, susceptibles enter an exposed phase (E). According to their age group, an exposed person exhibits clinical (symptomatic) infection (Ic) with probabilities for children, adults aged between 18 and 64 years, and adults aged 65 years and older, respectively; the other instances develop subclinical infection (Isc). Instances with fatal outcomes are transferred to compartment D, while cases with nonfatal outcomes are transferred to compartment R.

To complement the two-dose Moderna and Pfizer vaccines, a 25-day interval between the two doses of the primary vaccination was modeled. For all simulations, the team estimated the percentage reduction in overall clinical infections as well as total deaths between 1 January 2021 and 31 December 2021 in comparison to a no-vaccination case for five vaccine prioritization techniques: (1) prioritizing only adults aged over 65 years, (2) prioritizing only adults aged between 18 and 64 years with in-person work connections, (3) dividing priority vaccines uniformly between adults aged over 65 years and individuals working in person, (4) a 'tiered' method that prioritizes adults aged more than 65 years over HC workers, and (5) a 'tiered' method that prioritizes HC workers over adults aged over 65 years.

Results

In the simulation that used median parameter values, the tiered 65+ rollout lowered mortality by 31.32% compared to the no-vaccine case. This technique saved 25.13% more lives over prioritizing HC workers and 11.49% more lives than dividing prioritization among workers and adults aged over 65 years. This indicated that any distribution approach that provides early priority to older persons would reduce most deaths. There is also a slight decrease in clinical infections with a tiered rollout for those 65 and older.

In all scenarios, the tiered 65+ rollout was the superior strategy for reducing COVID-19-related mortality. All the prioritization scenarios reduced the proportion of clinical infections similarly. In 62.5% of all simulations, the tiered HC roll-out was the most effective strategy for minimizing clinical infections. On the other hand, the remaining 37.5% of simulations showed that the tiered 65+ roll-out was the most effective strategy. Vaccines attain their highest life-saving effectiveness within the first month, and the comparative benefit continues to climb through February. This illustrates that the scheduling of vaccines is crucial: prioritizing seniors reduces the greatest number of fatalities because they can establish vaccine-induced immunity before the outbreak peaks. In addition, tiered rollout techniques outperform their single prioritization counterparts during February and March after providing vaccines to the priority cohort.

In the simulation with median parameter draws, the effect of priority on the relative outcome for booster doses is smaller than for primary doses. Not only does a tiered 65+ rollout for boosters continue to save the most number of lives, but in contrast to the initial simulation, it also minimizes clinical infections the most. Although, as in the primary simulation, the decrease in clinical infections is about the same for all three strategies.

Overall, the study findings showed that the ideal technique for lowering mortality due to COVID-19 through vaccination was a tiered distribution of vaccines that prioritized individuals over 65 years of age, HC employees, with the remaining doses divided between LC adults aged between 18 and 64 years and children.

*Important notice

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.

Journal reference:
Bhavana Kunkalikar

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.

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