Mathematical modeling to inform vaccination strategies and testing approaches for COVID-19 in nursing homes

Abstract

Background: Nursing home residents and staff were included in the first phase of COVID-19 vaccination in the United States. Because the primary trial endpoint was vaccine efficacy (VE) against symptomatic disease, there are limited data on the extent to which vaccines protect against SARS-CoV-2 infection and the ability to infect others (infectiousness). Assumptions about VE against infection and infectiousness have implications for possible changes to infection prevention guidance for vaccinated populations, including testing strategies.

Methods: We use a stochastic agent-based SEIR model of a nursing home to simulate SARS-CoV-2 transmission. We model three scenarios, varying VE against infection, infectiousness, and symptoms, to understand the expected impact of vaccination in nursing homes, increasing staff vaccination coverage, and different screening testing strategies under each scenario.

Results: Increasing vaccination coverage in staff decreases total symptomatic cases in each scenario. When there is low VE against infection and infectiousness, increasing staff coverage reduces symptomatic cases among residents. If vaccination only protects against symptoms, but asymptomatic cases remain infectious, increased staff coverage increases symptomatic cases among residents through exposure to asymptomatic but infected staff. High frequency testing is needed to reduce total symptomatic cases if the vaccine has low efficacy against infection and infectiousness, or only protects against symptoms.

Conclusions: Encouraging staff vaccination is not only important for protecting staff, but might also reduce symptomatic cases in residents if a vaccine confers at least some protection against infection or infectiousness.

Summary: The extent of efficacy of SARS-CoV-2 vaccines against infection, infectiousness, or disease, impacts strategies for vaccination and testing in nursing homes. If vaccines confer some protection against infection or infectiousness, encouraging vaccination in staff may reduce symptomatic cases in residents.

Figure 1.

In each of the vaccine efficacy scenarios, increasing vaccination coverage among staff reduces total symptomatic cases (i.e. combined symptomatic cases among residents and staff). Two dose vaccinations are assumed to be completed before cases are able to be introduced into the facility starting on day 0.

Figure 2.

When considering symptomatic cases in residents, increasing vaccination coverage among staff from 0% reduces cases in vaccine scenarios 1 and 2, when vaccination confers at least low protection against infection and infectiousness. When the vaccine protects against symptoms only, increasing vaccination coverage among staff increases symptomatic cases among residents.

Figure 3.

The differences between testing strategies vary across vaccine efficacy scenarios. When the vaccine has low or no efficacy against infections and infectiousness (Scenarios 2 and 3), frequent screening testing is important for reducing total symptomatic cases in residents. Due to faster turnaround time, antigen testing results in lower incidence than PCR testing at the same frequency.