The duration of the pre-symptomatic stage was sampled from a Gamma distribution with a mean of 2.3 days. A proportion of infected individuals develop symptoms after a highly infectious pre-symptomatic stage. If infection occurred, the incubation period was sampled from a Gamma distribution with a mean of 5.2 days. Disease-specific parameters were sampled for each individual from their associated distributions and ranges. The infectivity of asymptomatic infection was assumed to be 26% relative to symptomatic infection, based on an average estimated 3.85 times higher incidence among close contacts of a symptomatic case compared to those of an asymptomatic individual. Based on the number of secondary cases generated during each stage of the disease, infectivity of mild and severe symptomatic stages was parameterized to be 44% and 89%, relative to the pre-symptomatic stage. For sensitivity analyses, we varied vaccine efficacy against infection, vaccine coverage, and the level of pre-existing immunity in the population.ĭisease transmission was implemented probabilistically for contacts between susceptible and infectious individuals in asymptomatic, pre-symptomatic, or symptomatic stages of the disease. Moreover, severity of symptoms and risk of death increase precipitously with age. This prioritization relies on the evidence that COVID-19 patients with pre-existing health conditions, including diabetes and hypertension, are 2–4 times more likely to develop severe disease than those without comorbidities. We explored a strategy where healthcare workers and high-risk individuals, including those with comorbidities associated with severe COVID-19 and individuals aged 65 and older, were prioritized for vaccination. To project the impact of vaccination and roll-out during ongoing outbreaks, we developed an age-structured transmission model, taking into account comorbidities and demographics of the US population. Implementation of vaccination programs will likely take several months, depending on the ability to roll out clinics and security of vaccine supply in each state. This compels an urgent need to understand the potential population-level impact of vaccination on COVID-19 transmission and disease outcomes. These vaccines have received emergency use authorization by the FDA, and vaccination has already started in the US with prioritization of healthcare workers, long-term care residents, and high-risk individuals. Results from two large efficacy trials (Pfizer - BioNTech, Moderna) indicate a vaccine efficacy of over 90% against symptomatic and severe disease, exceeding the preferred population-based efficacy specified by the World Health Organization and the United States (US) Food and Drug Administration (FDA). As of December 31, 2020, safety and efficacy results for a number of vaccines have been reported, and Phase III clinical trials for several other candidates are underway. To mitigate the mounting burden of COVID-19, vaccine development has occurred at an unprecedented pace. However, the vast majority of the global population remains susceptible to COVID-19, highlighting the need for an effective vaccine. Despite unprecedented movement restrictions, social distancing measures, and stay-at-home orders enacted in many countries, the COVID-19 pandemic has caused devastating morbidity and mortality.
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