Study: Physical distancing better at stemming COVID-19 than US/Mexico border closure

News brief

US Mexico border
Ben Amstutz / Flickr cc

A genomic analysis of SARS-CoV-2 in San Diego and Mexico reveals that physical distancing was more effective than international border closures in containing the virus.

Scripps Research scientists and colleagues sequenced more than 82,000 SARS-CoV-2 samples gathered from routine genomic surveillance in San Diego and the state of Baja California, Mexico, to reconstruct viral spread dynamics from March 2020 to the end of the first Omicron surge in December 2022.

The team then compared the genomes to those from elsewhere in the world and integrated air and land travel data from anonymous cellphone tracking. The US/Mexico border was closed to nonessential traffic from March 2020 to November 2021.

The results were published yesterday in Cell.

Mandate relaxation led to more distant travel

During stay-at-home and physical distancing mandates early in the pandemic, SARS-CoV-2 spread primarily within and between adjacent counties. But as mandates eased, people started to travel farther, and COVID-19 spread from distant locations rose.

"We found that the relative import risk from neighboring locations was low at the beginning of the pandemic, peaked during the spring of 2020, and decreased steadily from then until the end of available data (74.8% in April 2020 to 36.9% in July 2021)," the researchers wrote.

SARS-CoV-2 spread continuously between San Diego and Mexico throughout the pandemic, which the researchers said suggests that the partial US/Mexico border closure was ineffective.

We found that the relative import risk from neighboring locations was low at the beginning of the pandemic, peaked during the spring of 2020, and decreased steadily from then until the end of available data.

"After the mandates were relaxed, about 50% of the virus circulating in San Diego was the result of locally circulating virus, and 50% had been recently introduced as the result of travel between locations," first author Nathaniel Matteson, PhD, of Scripps Research, said in a news release.

The study authors said the results underscore the importance of collaborative interventions for pandemic prevention and containment. "We show that it's not necessarily how geographically close locations are to each other; the real measure is how connected two locations are in terms of the movement of people," co-senior author Mark Zeller, PhD, of Scripps Research, said in the release.

Higher mortality seen in patients with COVID-19, Pseudomonas coinfection

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Higher mortality was observed in patients with Pseudomonas aeruginosa (PA) bloodstream infection (PA-BSI) and COVID-19 co-infection, researchers reported today in Antimicrobial Stewardship & Healthcare Epidemiology.

Pseudomonas illustration
Jennifer Oosthuizen / CDC

Using data on patients in the Veterans Health Administration system, researchers from Case Western Reserve University assessed all-cause mortality in patients with PA-BSI and concomitant COVID-19 infection, comparing the pre-COVID-19 period (January 2009 to December 2019) with the COVID-19 period (January 2020 to June 2022). Patients in the COVID-19 period were further stratified by the presence or absence of COVID-19 coinfection. Previous studies have suggested a rise in PA-BSI incidence with the onset of the COVID-19 pandemic, but outcomes of patients with PA-BSI and COVID-19 coinfection have not been evaluated.

A total of 6,714 patients with PA-BSI were identified. Throughout the study period, PA resistance rates decreased. Thirty-day all-cause mortality decreased pre-COVID-19 and increased during COVID-19. Mortality was not significantly different between pre-COVID-19 (24.5%; 95% confidence interval [CI], 23.3 to 28.6) and the COVID-19 period/COVID-negative (26.0%; 95% CI, 23.5 to 28.6) patients, but it was significantly higher in COVID-19 period/COVID-positive patients (47.2%; 95% CI, 35.3 to 59.3).

After adjusting for potential confounders, the modified Poisson analysis showed that COVID-19 coinfection was associated with a 44% higher death rate (relative risk, 1.44; 95% CI 1.01 to 2.06). Higher Charlson Comorbidity Index, higher modified Acute Physiology and Chronic Health Evaluation score, and no targeted PA-BSI treatment within 48 hours were also predictors of higher mortality.

Potential synergy between the 2 infections

The study authors suggest several factors could account for increased mortality among patients coinfected with PA-BSI and COVID-19, including pandemic-related staffing shortages and limited availability of infectious diseases expertise. But they also say further research is needed on the interaction between the two infectious processes.

"The relationship between COVID-19 and PA should be explored in more detail to determine whether characteristics of either pathogen enhance the infectivity of the other or work synergistically against the host immune response, and inclusion of observations of patients with COVID-19 with and without PA-BSI could be helpful to this end," they wrote. 

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