Health

Non-pharmaceutical interventions effectively reduce COVID case burden in the United States

The coronavirus disease 2019 (COVID-19) pandemic was a profoundly disrupting influence worldwide that has caused devastating effects on human health and economic stability. In the absence of any specific antiviral measures, the focus has been on attempting to contain viral spread by non-pharmaceutical interventions (NPIs). A new study published on the preprint server medRxiv* analyzes the impact of such measures in the United States.

Study: Non-Pharmaceutical Interventions and COVID-19 Burden in the United States. Image Credit: Corona Borealis Studio / Shutterstock.com

Background

Various NPIs have been used over the last century in a number of local and global viral outbreaks. Throughout the current COVID-19 pandemic, the United States adopted multiple interventions such as travel restrictions, declaration of an emergency, social distancing, self-quarantine advisories, and mask recommendations.

However, there is limited evidence for the efficacy of such NPIs on respiratory illnesses. Most available data refer to individual NPIs rather than multiple. The current study aimed to assess how these steps, both when they were implemented in a state-specific manner and after they were relaxed, affected the incidence of COVID-19 in the country.

About the study

With a retrospective observational design, the researchers covered the U.S. population between January 19, 2020, which is when the first American case was identified, and March 7, 2021.

The scientists determined that during this period, there were approximately 28.6 million cases of COVID-19 that resulted in about 512,000 deaths. More than 400 NPIs were adopted and stopped over this period.

Case breakpoints were identified, where the case velocity changed in relation to NPI adoption or discontinuation. The highest number of breakpoints was one per week. Overall, there were about 600 breakpoints, with a decrease in 433 and an increase in 170. The median number of case breakpoints was 12 for all the states, ranging from 7-17.

Study findings

It appears that with the use or relaxation of shelter-in-place orders or mask use in public, the odds of a reduction in case velocity are doubled. With a ban on indoor restaurant dining, the odds are increased by 62%.

Comparably, with bans on indoor gatherings of less than 10 people, the odds of reduced case velocity increase by almost 70%. A mild ban limiting gathering to more than 10 people was not as effective, as it was associated with a 50% increase in the odds of a rapid decrease in cases.

After adjusting for other NPIs, the use of shelter-in-place orders led to 50% greater odds of decreasing case velocity, while with public mask mandates, the reduction was more 2.3 times more likely. A mild ban on indoor public gatherings led to a 50% increase in case velocities.

Breakpoints leading to increased death velocity occurred at 461 points. About 72% of these breakpoints were related to a reduction in case velocity, while 28% were related to an increase.

The adjusted odds of reduced death velocity with a shelter-in-place order were 90% higher. The unadjusted odds were doubled with this NPI, while the odds of a decrease were 50% higher with a ban on indoor dining or on the indoor public gatherings of fewer than 10 people.

Implications

The researchers found correlations between the number of NPIs and a decrease in the number of new cases and deaths in corresponding periods of time. The adjusted models show that new cases were diagnosed far less frequently following the implementation of shelter-in-place orders and public mask mandates.

In fact, mask use in public was related to more than twice the chance of containing the transmission of the virus even, after allowing for the effects of other NPIs. The effect of this single measure may be due to other behavioral changes that accompany the use of a mask in public, as well as the effect of the face barrier on viral transmission.

Masks are key in interrupting the aerosol spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, according to the current understanding of viral transmission. As such, these findings support public health guidance for mask use in public during respiratory virus outbreaks.

Restricting gatherings indoors to more than ten people, not of the same household led to an increase in new cases, thereby demonstrating that this measure was inefficient in limiting spread. In other words, NPIs that limit large public gatherings are more effective.

Shelter-in-place orders were the only measure that effectively reduced COVID-19 deaths. It may be that decreased numbers of cases occurred mostly among younger people with a low risk of death, thus accounting for the apparent mismatch between reduced case numbers and mortality rates.

Some earlier studies diverged from the conclusions of this study, showing that more restrictive NPIs did not reduce viral spread better than less restrictive ones. However, these studies were small in size and did not account for diversity.

In contrast, the current study shows that limiting public indoor gatherings to less than ten changes both the fraction of reduction in case numbers and ensures restricted transmission. Multiple NPIs were highly efficient at reducing the number of new cases after adjusting for simultaneous effects.

“[The current study] is supportive of prior expert opinions encouraging early, sustained, and layered application of NPIs to mitigate consequences of pandemic viral disease.”

*Important notice

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

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