The COVID-19 pandemic has taken a profound toll on human life and the global economy, along with a devastating social and psychological impact due to the recurrent lockdown situations. Recently initiated COVID-19 vaccination programs are intended to prevent deaths and reduce peaks in hospitalizations to avoid the collapse of healthcare systems.  Since many infected people develop severe disease and debilitating long-term symptoms, limiting the cumulative number of cases is a priority.

Another primary goal of vaccination is to allow the safe reopening of businesses, schools, and public spaces. Prior to implementing vaccination, several strategies such as mask usage, testing and tracing, and social distancing were in place to prevent a surge in cases and subsequent shutdowns. While these measures helped control the transmission of the virus, they were insufficient to prevent the recurring waves of infection in many countries.

Another major concern is the emergence of more infectious variants of the virus in many countries such as the UK, Brazil, and South Africa. The UK variant or B.1.1.7 variant is believed to be significantly more contagious and is overwhelming health care systems in the UK as well as spreading across the globe.

While the impact of non-pharmaceutical interventions on new SARS-CoV-2 variants is still not clear, these interventions come at a significant cost. School and business closures disproportionately impact people of color and socioeconomically backward regions. A key goal of vaccination plans should be to limit the intensity and duration of lockdowns this year.

Analyzing the complexities of vaccine implementation using mathematical models

Recently, researchers from France and the USA analyzed the complexities of vaccine implementation during the 3^rd wave of SARS-CoV-2 infections and potentially future waves, using a mathematical model calibrated to data from King County, Washington. The researchers' goal was to understand which of the variables have the greatest impact on infections and mortality to help limit them while decreasing time under lockdown. The study is published on the preprint server medRxiv*.

The model used by the team allows for projections in other states with lower or higher incidence at the time of initiation of local vaccination programs. The researchers simulated various scenarios with different vaccination rates, vaccine efficacy profiles, and case thresholds for implementing and relaxing partial lockdowns and assumed that a more contagious variant is currently present at mild levels.

"Our model projects that with the new more infectious variants, higher case thresholds for triggering partial lockdown will result in greater numbers of total infections and deaths per capita."

High vaccination rate and low case threshold for triggering partial lockdown are the key variables for limiting total infections and deaths regardless of vaccine efficacy profile. Heat maps demonstrating joint effects of vaccination rate (x-axis) and case threshold for triggering partial lockdown (y-axis) are shown for four plausible vaccine profiles. a. VESUSC=90% / VESYMP=10% / VEINF=10%, b. VESUSC=50% / VESYMP=10% / VEINF=10%, c. VESUSC=10% / VESYMP=90% / VEINF=10%, d. VESUSC=10% / VESYMP=50% / VEINF=10%. Outcomes are total infections (top row), total deaths (middle row) and days under partial lockdown after vaccination initiation (bottom row). Increasing vaccination rate lowers infections and deaths across all scenarios. Increasing vaccination rate substantially lowers total days under lockdown, particularly when case threshold trigger for partial lockdown is low. Lowering case thresholds for triggering partial lockdown decreases total infections and deaths but results in higher number of days under partial lockdown in many scenarios. A decrease in VESUSC (a to b) results in more infections and deaths with only slight impact on time under lockdown. A decrease in VESYMP (c to d) results in more infections and deaths with only slight impact on time under lockdown. VESUSC provides a substantial reduction in infections but slight reduction in deaths relative to an equivalent VESYMP (a to c and c to d).

Increase in vaccination rate decreases the number of cases and deaths, and the number of lockdown days

In all the scenarios, the new variant rapidly became dominant by early summer. Low case thresholds for partial lockdowns during current and future waves strongly predict lower numbers of infections, hospitalizations, and deaths in 2021. However, there is a predicted delay in the onset of a surge in new variant-related infections in places with relatively higher seroprevalence. For all vaccine efficacy profiles considered in this study, increasing the vaccination rate decreases the number of infections and deaths and the number of days under partial lockdown.

"High efficacy of vaccines against new more contagious variants, and in particular their ability to block ongoing transmission rather than simply preventing symptoms, will potentially prevent thousands of infections and save hundreds of lives in King County."

The researchers project significant uncertainty in future waves' timing and intensity due to variable estimates of infectiousness of newly emerging variants, the efficacy of vaccines against these new variants, refusal of vaccine, and future adherence to masking and social distancing of SARS-CoV-2 infection. Nevertheless, in all plausible scenarios considered in this work, rapid vaccination and early implementation of lockdown are the most crucial variables that help save the most number lives.

"It is often stated that vaccines do not save lives, vaccinations do. Our modeling strongly reinforces this point."

*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.

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.