The COVID-19 pandemic has unleashed devastating health and economic crises worldwide, causing more than 3.9 million deaths and 183 million reported infections globally.1 While the United States has accounted for more than 600,000 deaths, it also has supported the development of highly efficacious vaccines, granting emergency authorizations and delivering the products at an unprecedented pace. As of July 2, the U.S. had administered more than 328 million vaccine doses, with 67 percent of adults having received at least one dose.2,3 The number of cases has fallen from more than 300,000 per day at the apex of the pandemic in January 2021 to less than 20,000 per day in mid-June.
The precipitous decline in U.S. cases is especially impressive as more transmissible variants have emerged in recent months, including the B.1.1.7 (Alpha), P.1 (Gamma), and B.1.617.2 (Delta) variants. The Alpha variant, first identified in the United Kingdom, is 50 percent more contagious than the original COVID-19 variant,4 with higher mortality risk.5 The Gamma variant, initially detected in Brazil and imported to the U.S. in January 2021, became one of the dominant variants by mid-May.6 Ominously, the Delta variant, linked to a resurgence of COVID-19 infections in India, Nepal, and other southeast Asian countries, is threatening to shift the course of the pandemic in the U.S. With an even higher transmissibility than the Alpha variant,7 the Delta variant currently accounts for more than 40 percent of positive tests and is already establishing dominance in some U.S. states.2,6
The efficacy and safety of authorized vaccines against the original viral variant are well established based on randomized controlled trials showing that they prevent symptomatic and severe disease.8–10 However, the effectiveness of the U.S. vaccination campaign in reducing COVID-19 hospitalizations and deaths in the face of emerging highly transmissible variants has not yet been fully evaluated.
To assess the impact of the U.S. vaccination program, we expanded our age-stratified, agent-based model of COVID-19 to include transmission dynamics of the Alpha, Gamma, and Delta variants in addition to the original Wuhan-1 variant.11 (For details, see How We Conducted This Study). Briefly, the model compared the observed epidemiologic trajectory (cases, hospitalizations, and deaths) to two counterfactual scenarios, one in which no vaccination program occurred and another under which daily vaccinations were administered at only half the actual daily pace.
- Without a vaccination program, by the end of June 2021 there would have been approximately 279,000 additional deaths and up to 1.25 million additional hospitalizations.
- If the U.S. had achieved only half the actual pace of vaccination, there would have been nearly 121,000 additional deaths and more than 450,000 additional hospitalizations.
- If there had been no vaccination program, daily deaths from COVID-19 potentially would have jumped to nearly 4,500 deaths per day during a second “2021 spring surge” — eclipsing the observed daily peak of 4,000 during the first 2021 winter surge.
The vaccination campaign markedly curbed the U.S. pandemic. If there had been no COVID-19 vaccination program, daily deaths from COVID-19 would have created a second wave (a “spring surge”) — of nearly 4,500 deaths per day — potentially larger than the first wave of the year, which peaked at 4,000 deaths per day in January. Most of the additional deaths during the second 2021 wave (the shaded area of the exhibit here) would have occurred because of an increase and spread of the more transmissible Alpha variant.
This exhibit shows that if the pace of vaccinations each day had been only half of what was actually achieved, the daily rate of deaths would still have exceeded the observed rate of deaths.
The exhibit above compares the cumulative number of averted deaths (in thousands) under the two counterfactual scenarios. In the absence of a vaccination program, we estimate that there would have been approximately 279,000 additional deaths (above the approximately 304,000 deaths recorded since vaccination began). If the vaccination program had achieved only half the daily pace there would have been nearly 121,000 additional deaths compared to what actually occurred.
The number of hospitalizations would have been markedly higher if the vaccination program had been less effective. This exhibit shows that without a vaccination program there would have been more than 1.2 million additional hospitalizations during the period (above the approximately 1.5 million hospitalizations recorded since the vaccination program began). With a vaccination program achieving only half the pace of vaccination there would have been more than 450,000 additional hospitalizations.
The number of COVID-19 cases would have been substantially higher under both alternative scenarios, as this exhibit shows. There would have been an additional 26 million cases in the absence of a vaccination program, or nearly 22 million additional cases if the pace had been half as effective as it actually was. The smaller difference in the number of averted cases under the two scenarios compared to the difference in the number of deaths or hospitalizations most likely reflects the added effectiveness of the vaccine at reducing severe disease and death among vaccinated individuals (in addition to its potential ability to reduce transmission).
The United States reported the highest daily COVID-19 cases worldwide for much of 2020 and the beginning of 2021.12 Since the start of the U.S. vaccination program in December, more than 303,000 Americans have died and more than 1.5 million have been hospitalized. The swift early rollout of the vaccine program, which ramped up during February and March and exceeded 3.3 million doses administered per day in April 2021, played a critical role in curtailing the pandemic.
Our results demonstrate the extraordinary impact of rapidly vaccinating a large share of the population to prevent hospitalizations and deaths. The speed of vaccination seems to have prevented another potential wave of the U.S. pandemic in April that might otherwise have been triggered by the Alpha and Gamma variants. Additional new variants such as Delta will pose a special threat to unvaccinated populations in coming months. A renewed commitment to expanding vaccine access will be crucial to achieving higher levels of vaccination necessary to control of the pandemic and prevent avoidable suffering, particularly for those in historically underserved groups and areas of the U.S. with low vaccination rates.