Modeling study shows non-pharma interventions beneficial until at least 75% vaccine coverage

As of May 29, the U.S. hit a 41% Covid-19 vaccination rate—but despite this positive trajectory, people need to keep wearing masks, maintain social distance, and minimize time outside the house, according to a modeling study.

Based on an analysis of data on Covid-19 spread in North Carolina (n=10.5 million population), approximately 1.8 million infections and 8,000 deaths could be prevented during 11 months with more efficacious vaccines, higher vaccination coverage, and maintaining non-pharmaceutical interventions (NPIs), such as distancing and use of face masks, reported Mehul D. Patel, PhD, of the University of North Carolina at Chapel Hill, and co-authors.

They calculated that, in the worst-case vaccination scenario of 50% efficacy and 25% coverage, a mean of about 2.2 million new infections occurred after vaccination began with NPIs removed, and a mean of about 800,000 new infections occurred with NPIs maintained during 11 months.

The “findings highlight the importance of continued adherence to NPIs while the population is vaccinated, particularly under scenarios of lower vaccine efficacy and coverage,” they wrote in JAMA Network Open.

Many states have begun easing or removing pandemic-related restrictions. For instance, Florida officials suspended Covid-19 restrictions on businesses or individuals in early May, while California has set up a color-coded system that will determine to what degree counties can partially or fully “open.”

Some health experts have warned that it is too soon to lift pandemic restrictions, especially given the rise in transmissible variants. Not surprisingly, some economists have argued in favor of lifting restrictions in order to jump start the battered U.S. economy.

Patel and co-authors stressed that “sufficient population protection to slow new infections was achieved through vaccination during 6 months and NPIs… Our findings suggest that coordinated efforts are needed to maximize vaccine coverage and adherence to NPIs to reduce Covid-19 burden to a level that could safely allow a resumption of many economic, educational, and social activities.”

They created a decision analytical study model to estimate the effect of hypothetical scenarios of vaccine efficacy and population coverage on SARS-CoV-2 infections and Covid-19-related hospitalizations and deaths in North Carolina.

The authors compared the outcomes of varying efficacy and coverage with NPIs maintained and removed concurrently with vaccine distribution. The model simulated Covid-19 transmission and progressions from March 24, 2020, to Sept 23, 2021.

The North Carolina cohort had 18% who were ages ≥65, and 20% who were Black, while 19% of households earned less that the poverty threshold set by the U.S. Census Bureau. Most of the cohort was rural (n=80/100 counties). The authors noted that Asian, Native American, and multiracial ethnicities were not stratified due to small denominators.

In the best-case scenario of 90% efficacy and 75% coverage, maintaining NPIs resulted in a mean of about 451,000 new infections, for a 19% absolute risk reduction versus a mean of around 528,000 new infections with NPIs removed.

“With NPIs removed, lower efficacy (50%) and higher coverage (75%) reduced infection risk by a greater magnitude than higher efficacy (90%) and lower coverage (25%) compared with the worst-case scenario,” for a mean absolute risk reduction of 13% and 8%, respectively, the authors wrote.

They also reported that all vaccination scenarios with NPIs had lower risks than without NPIs, with maintaining them in the worst-case vaccination scheme reducing infections by a mean of 15%, and by 19% in the best-case vaccination schemes plus NPIs.

“Similar patterns for the joint association of vaccination and NPIs were observed for mortality risks and hospitalizations,” the authors noted. Not surprisingly, racial/ethnicity modeling found white individuals had the lowest incidence of infections, with Hispanic individuals demonstrating the lowest hospitalization and mortality rates.

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Black individuals tended to have the highest incidence of infections, hospitalizations, and mortality, Patel’s group reported, which is in line with data showing that Black communities have been particularly burdened by Covid-19.

The authors also found that rural counties showed a higher incidence of infections, hospitalizations, and deaths compared with urban tracts in all scenarios. While urban and suburban areas experienced comparable cumulative infections in the model, suburban areas had more hospitalizations and higher mortality, they noted.

Study limitations included the unique characteristics of North Carolina that may not translate to other states or regions.

In an invited commentary accompanying the study, Alexander Doroshenko, MD, MPH, of the University of Alberta in Edmonton, Alberta, Canada, wrote that it was “important to recognize that NPIs and vaccination minimize the impact of the pandemic through distinct mechanisms: the former decrease the effective transmission rates, and the latter increases the number of people who are nonsusceptible to infection or to severe outcomes of infection in the population.”

“In the short and medium term, continuing NPIs will likely have an effect that is complimentary or even synergistic to vaccination in the effort to end the Covid-19 pandemic,” he wrote.

Doroshenko also wrote that the “findings of the disproportionate burden of Covid-19 among ethnic/racial minority populations coupled with evidence of increased vaccine hesitancy among the same groups,” should led to more research on “how lifting NPIs can motivate individuals to get vaccinated as discussed in the Centers for Disease Control and Prevention recommendations.”

  1. Removing nonpharmaceutical interventions (NPIs) while vaccines were distributed resulted in substantial increases in SARS-CoV-2 infections., hospitalizations, and deaths from Covid-19, according to a decision analytical model of North Carolina.

  2. As NPIs were removed, higher vaccination coverage with less efficacious vaccines contributed to a larger reduction in risk of infection versus with more efficacious vaccines at lower coverage.

Shalmali Pal, Contributing Writer, BreakingMED™

Patel reported support from the National Center for Advancing Translational Sciences (NCATS), the Council of State and Territorial Epidemiologists (CSTE), and the CDC. Co-authors reported support from the National Institute of Allergy and Infectious Diseases, Translational and Clinical Sciences, CDC-Prime, University of North Carolina/North Carolina State Translational Research Grant, NCATS, CTSE, and the Department of Health and Human Services.

Doroshenko reported a relationship with Sanofi Pasteur/University of Alberta/Canadian Institutes of Health Research, and support from Public Health Agency of Canada, Alberta Health, and the University of Alberta Hospital Foundation.

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