In a recent study posted to the medRxiv* preprint server, researchers assessed the effect of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination in the US.
By August 1st 2022, coronavirus disease 2019 (COVID-19) had been responsible for nearly 90 million SARS-CoV-2 infections and one million fatalities in the US. SARS-CoV-2 vaccinations have been a crucial part of the US pandemic response since December 2020. However, the effects of immunization are difficult to measure.
About the study
In the present study, researchers calculated the number of COVID-19 cases, hospitalizations, and fatalities prevented by SARS-CoV-2 vaccination within the first six months after the vaccine became available using a dynamic county-scale metapopulation model.
The team combined a data assimilation technique with a metapopulation model coupled with a Susceptible-Exposed-Infected-Recovered (SEIR) structure. Here, the spread of SARS-CoV-2 throughout and among the 3142 American counties was simulated. The reported case counts for each American county from the time the first COVID-19 cases were discovered there in February 2020 through the date of the first authorized SARS-CoV-2 vaccination on December 14th, 2020, were assessed using the model-inference system.
The team compared the baseline vaccination scenario to three counterfactual no-vaccination scenarios that were simulated over the same period to measure the burden prevented by vaccination. To simulate various levels of non-pharmaceutical intervention (NPI) response in the absence of vaccination, all counterfactual scenarios assumed no vaccinations but had varied transmissibility:
- Counterfactual Scenario 1: A scenario in which there was no transmission change and no vaccination, and each location’s time-varying reproductive number (Rt) and daily time series were as implied by the baseline scenario;
- Counterfactual Scenario 2: A scenario in which there is no vaccination and Rt for each location-day is 10% higher than in the baseline scenario; and
- Counterfactual Scenario 3: An absence of vaccination, with a 10% reduction in Rt for each location day compared to the baseline scenario.
The study assumed that excess cases would have persisted and resulted in hospitalizations and mortalities at the same overall rate as observed in each US state in the summer and fall of 2020 before the vaccine was available. This assumption allowed the estimation of hospitalizations and deaths in the counterfactual scenarios. Each scenario’s total number of prevented cases was multiplied by a state-specific pre-vaccine Case Fatality Rate (CFR) and Case Hospitalization Rate (CHR).
The team used data from the US-based Premier Healthcare COVID-19 claims database to multiply the distribution of estimated COVID-19-related hospitalizations prevented by the distribution of expenses per hospitalization episode.
On December 14th, 2020, the beginning of the simulation period, it was predicted that 74.1% of the American population was susceptible, while 0.8% had been exposed, 0.8% had become infectious, and 24.3% had recovered. The proportion of the population that had not yet contracted COVID-19 since the pandemic started, known as the median estimated susceptible fraction, ranged between 58% in North Dakota and 94% in Vermont. In states located in the northwest and northeast, the susceptible fraction was the highest. In Minnesota, the median time-dependent reproductive number was 0.8, while in Tennessee, it was 2.0.
Before immunization, the CFR ranged between 0.5% in Alaska and 2.3% in Rhode Island. The CHR was between 3.8% in Alaska and 20.7% in Kentucky during the same time frame. Despite a slight decrease in CHR at the national level from the pre-vaccination period to the analysis period, the team found no consistent population-level changes in CFR at the national level or in CFR and CHR at the state level.
The baseline model predicted a total of 16.1 million COVID-19 cases, 1.4 million related hospitalizations, and over 246 thousand deaths between December 14th and June 3rd, 2021. Almost 51% of Americans had received a minimum of one COVID-19 vaccine dose by June 4th, 2021. Location-specific vaccination rates varied greatly, from 35% in Mississippi to 74% in Vermont. The number of vaccinations received by individuals each week increased over time, starting at fewer than five million per week and peaking at 14 million in April when vaccination was made available to all adults aged 16 years and older.
The majority of prevented cases were recorded between April and June 2021 in the three counterfactual scenarios. The median estimates of cases prevented for each state ranged from 1000 to 6400 cases per 100,000 people. Hospitalizations prevented ranged from 74 to 752 median cumulatively per 100,000, while deaths prevented ranged from 16 to 128 median cumulatively per 100,000. At the start of the immunization campaign, a greater vaccination rate and population susceptibility were associated with a greater averted case burden.
Overall, the study findings demonstrated that COVID-19 vaccination decreased the disease burden. The COVID-19 vaccination was linked to over 8 million fewer confirmed cases, over 120,000 fewer deaths, and 700,000 fewer hospitalizations within the first six months of the campaign, according to base case statistics. As a result, the COVID-19 vaccines were a vital part of the US’s public health response to the COVID-19 epidemic.
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.