When can we stop wearing face masks?

The global launch of Coronavirus Disease Prevention 2019 (COVID-19) vaccines is underway. Nine different vaccines are approved for distribution by different countries. With four vaccines approved for sale in the US, 29.9% of the US population were fully vaccinated against SARS-CoV-2 as of April 30, 2021.

Even with ongoing vaccination programs, reports suggest spikes in COVID-19 cases in some U.S. states. Despite these increased case numbers, several US states have recently lifted policies that make non-pharmaceutical interventions mandatory in public spaces and appear to have no intention of resuming the interventions.

Epidemiological models of the impact of the vaccine rollout on the transmission of SARS-CoV-2 show that discontinuing non-pharmaceutical interventions in the early stages of the vaccine rollout results in a later increase in COVID-19 cases, leading to hospitalizations and death.

There is growing concern that individuals will begin to avoid non-pharmaceutical measures to prevent SARS-CoV-2 transmission and attempt to return to pre-pandemic normalcy before vaccine coverage can effectively minimize the risk of transmission.

Recent models in the US show that lifting non-pharmaceutical interventions too early leads to a locally increased increase in the number of COVID-19 cases. However, the effects of non-pharmaceutical interventions on individual risk of SARS-CoV-2 infection in partially and fully vaccinated populations are unclear.

Use of an agent-based model to simulate SARS-CoV-2 transmission during indoor gatherings

Researchers from the United States recently used a previously published agent-based model (ABM) to simulate indoor SARS-CoV-2 transmission, including compilations of different population densities, duration, and vaccination status. The study will be published on the preprint server medRxiv *.

The study assumed that infectious individuals are asymptomatic in indoor gatherings because people with symptoms deliberately stay away from such gatherings and people with partial immunity do not attend gatherings.

At low population densities and thresholds for recording duration, non-pharmaceutical interventions to prevent infections and increased vaccination rates reduce the likelihood of observing ≥ 1 successful SARS-CoV-2 transmission events in simulations.

At low population densities and thresholds for recording duration, non-pharmaceutical interventions to prevent infections and increased vaccination rates reduce the likelihood of observing ≥ 1 successful SARS-CoV-2 transmission events in simulations.

The results do not show a final vaccination rate that eliminates the need for non-pharmaceutical interventions

Using the ABM model, researchers were able to quantify the difference in risk of SARS-CoV-2 infection in the presence and absence of non-pharmaceutical interventions.

“We previously used this model to quantify the impact of non-pharmaceutical interventions on reducing the risk of SARS-CoV-2 transmission during an overarching indoor event.”

They found that non-pharmaceutical preventive measures often lower secondary attack rates, especially with short interactions. As a result, there is no definitive vaccination protection that eliminates the need for non-pharmaceutical prevention strategies.

However, they also found that reducing the risk of SARS-CoV-2 infection through non-pharmaceutical interventions could be proportional to the prevalence of COVID-19. As COVID-19 prevalence decreases in the US, non-pharmaceutical interventions may still provide protective effects, but the potential benefit of the interventions may be small enough to be considered “effectively negligible.”

“It is clear that when the population is low, a two-meter social distancing provides additional protection when used in conjunction with fabric face coverings, even for relatively long gatherings.”

The results may not reflect real-world situations as the study is based on simulated situations and behaviors

The authors warn that the study results must be viewed in the context of simulated situations and behaviors and may not reflect the circumstances that exist during real transmission events. They added that while this is quite common in ABM-based studies, they believe their model is accurate enough to highlight general trends in SARS-CoV-2 transmission and the risk of infection indoors.

In summary, the results of this study show that non-pharmaceutical interventions are able to lower secondary attack rates, mainly during brief interactions. As a result, there is no definitive vaccine protection that could potentially completely eliminate the need for non-pharmaceutical interventions. In addition, the beneficial effect of these interventions on reducing SARS-CoV-2 infection in indoor gatherings may depend on COVID-19 prevalence.

If the US COVID-19 prevalence declines in the future, non-pharmaceutical interventions will likely still have protective effects, but the potential benefit may be small enough to stay within “effectively negligible” risk thresholds.

* Important NOTE

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice / health-related behavior, or be treated as established information.

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