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People who live in crowded or poorer areas are more likely to test positive for COVID-19, according to a UK study of infection in the general population.

Chronic kidney disease may also be linked to the viral illness, according to the report published Friday in The Lancet Infectious Diseases journal that analyzed results from about 3,800 people tested for Covid-19 in the UK

As the coronavirus has spread around the globe, most research has focused on patients hospitalized with severe, sometimes life-threatening infections. The study delved into a grey area - people treated by general practitioners and primary care doctors who may be quite ill, but were seen outside hospitals or emergency rooms.

"It's important to know which groups in the wider community are most at risk of infection so that we can better understand SARS-CoV-2 transmission and how to prevent new cases," Simon de Lusignan, a professor at the University of Oxford and director of the Royal College of General Practitioners surveillance center in the U.K., said in a statement.

The study also found the infection was more likely in people who are older, obese, black or male, characteristics that have emerged in earlier research.

Smoking was linked to a lower likelihood of a positive test result, but that doesn't mean the practice offers protection against infection, the authors said. Among the possible explanations: smoking may hamper the sensitivity of the test or smokers may be more likely than nonsmokers to have an ongoing cough and get tested for the virus. Smoking could increase the severity of Covid-19, they said.

The observational study, conducted between Jan. 28 and April 4, relied on electronic records from doctors' practices across England. The authors cautioned that there were small numbers of people from ethnic groups in the study. More research is needed to understand other factors that may be linked with infection, such as high-risk jobs, education and income, along with differences in access to health care and testing among groups, they wrote.