In a recently published study medRxiv* Preprint servers, researchers evaluated the effect of vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the United States.
Until August 1stSt In 2022, the coronavirus disease 2019 (COVID-19) was responsible for nearly 90 million SARS-CoV-2 infections and one million deaths in the United States. SARS-CoV-2 vaccinations have been a critical part of the US pandemic response since December 2020. However, the effects of vaccination are difficult to measure.
About the study
In the present study, researchers calculated the number of COVID-19 cases, hospitalizations, and deaths prevented by SARS-CoV-2 vaccination within the first six months of vaccine availability using a county-level dynamic 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 in and among the 3142 American counties was simulated. The reported number of cases for each US county from the date of detection of the first COVID-19 cases there in February 2020 to the date of the first approved SARS-CoV-2 vaccination on December 14th2020, were evaluated using the model inference system.
The team compared the baseline vaccination scenario to three counterfactual no-vaccination scenarios simulated over the same period to measure the burden prevented by vaccination. To simulate different levels of response to non-vaccination non-pharmaceutical interventions (NPI), all counterfactual scenarios assumed no vaccination but had different transferability:
- Counterfactual Scenario 1: A scenario in which there was no transmission change and no vaccination and the time-dependent reproductive number of each site (Rt) and the daily time series corresponded to the base case;
- Counterfactual scenario 2: A scenario where there is no vaccination and Rt 10% higher than baseline for each site day; and
- Counterfactual Scenario 3: Missing vaccination with a 10 percent reduction in Rt for each site day compared to the baseline scenario.
The study hypothesized that the excess number of cases would have persisted, leading to hospitalizations and deaths at the same overall rate observed in every US state in the summer and fall of 2020 before the vaccine became available. This assumption allowed the estimation of hospital admissions and deaths in the counterfactual scenarios. The total number of prevented cases of each scenario was multiplied by a country-specific case fatality rate (CFR) and pre-vaccination hospitalization rate (CHR).
The team used data from Premier Healthcare’s COVID-19 claims database in the United States to multiply the distribution of estimated COVID-19-related hospitalizations prevented by the cost-per-hospitalization cost distribution.
On December 14ththAs of January 1, 2020, the start of the simulation period, 74.1% of the American population was predicted to be susceptible, while 0.8% had been exposed, 0.8% had become infectious, and 24.3% had recovered. The proportion of the population who have not yet contracted COVID-19 since the beginning of the pandemic, known as the median estimated susceptible fraction, ranged from 58% in North Dakota to 94% in Vermont. The states located in the Northwest and Northeast had the highest proportion susceptible. In Minnesota, the mean time-dependent reproduction number was 0.8, while in Tennessee it was 2.0.
Before immunization, the CFR ranged from 0.5% in Alaska to 2.3% in Rhode Island. The CHR ranged from 3.8% in Alaska to 20.7% in Kentucky over the same period. Despite a slight decrease in national-level CHR from the pre-vaccination period to the analysis period, the team found no consistent changes in national-level CFR or state-level CFR and CHR at the population level.
The base model projected 16.1 million total COVID-19 cases, 1.4 million related hospitalizations and over 246,000 deaths between December 14th and June 3rdrd, 2021. Nearly 51% of Americans had received at least one COVID-19 vaccine dose as of June 4thth, 2021. Site-specific immunization rates varied widely, from 35% in Mississippi to 74% in Vermont. The number of vaccines individuals receive each week increased over time, starting at less than five million per week and peaking at 14 million in April, when the vaccine became available to all adults aged 16 and over.
The majority of prevented cases were recorded in the three counterfactual scenarios between April and June 2021. The median estimates of prevented cases for each state ranged from 1,000 to 6,400 cases per 100,000 people. Median hospitalizations prevented ranged from 74 to 752 per 100,000, while median deaths prevented ranged from 16 to 128 per 100,000. At the beginning of the vaccination campaign, higher vaccination coverage and greater population susceptibility were associated with a greater caseload averted.
Overall, the study results showed that the COVID-19 vaccination reduced the burden of the disease. The COVID-19 vaccination was associated with over 8 million fewer confirmed cases, over 120,000 fewer deaths and 700,000 fewer hospital admissions within the first six months of the campaign, according to base case statistics. As a result, the COVID-19 vaccines have been an integral part of the US public health response to the COVID-19 epidemic.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be relied upon as conclusive, guide clinical practice/health behavior, or be treated as established information.