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Both rural and urban areas of Mali experienced high seroprevalence of COVID-19, but a low burden of symptomatic disease, a researcher said.

From spring to fall 2020, the rate of seropositivity among those with self-reported symptoms jumped from 20.8% to 48.6%, while those who reported symptoms and were seronegative also increased from 9.8% to 49.3% in the cohort, reported John Woodford, MD, of the National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, Maryland.

However, during March to July, the COVID-attributable fraction was 11%, and from August to December, the COVID-attributable fraction was 0%, he said in a presentation at the American Society of Tropical Medicine & Hygiene (ASTMH) virtual meeting.

Moreover, the percentage of seropositive people in the cohort with symptomatic illness above the background illness reporting rate was 0%-11%, far below the age-adjusted 33% of cases expected, based on the U.S. rate of cases.

There were only three hospitalizations throughout spring and fall 2020, which also fell far below the expected 11-30 hospitalizations, and no deaths.

Woodford noted there was "a lot of anecdotal reports that there was a limited disease burden in Mali."

"It was repeated over and over again for a variety of sub-Saharan African settings, but there's very limited data to back this up," he added.

His group performed a longitudinal multi-center cohort serosurvey of both urban and rural areas of Mali, using two-antigen ELISA qualified for use in the country. Participants were also given a questionnaire to fill in self-reported symptoms, medical, and social history.

Overall, they obtained serosurvey data from 3,671 participants at four sites, who were a median age of 15. They noted the dates of the symptom questionnaires were March to July 2020 and August to December 2020.

No symptoms were independently associated with seropositivity, Woodford said. Seropositive people were also not more likely to be absent from work, seek medical care, or be hospitalized compared to seronegative people in the cohort during spring, but more likely to seek medical care in the fall than seronegative people (63.4% vs 45.9%, respectively).

Examining long COVID, which was defined as prolonged symptoms more than 28 days later, six participants reported post-COVID sequelae. Four of six were seropositive, and all were females ages 14-58. However, the four people with prolonged symptoms fell far below the expected age-adjusted estimate of 72 people, based on U.S. and U.K. data.

Woodford noted that the second follow-up visit was during the malaria season, when there was a high percentage of seropositivity, but the proportion of those with self-reported symptoms was comparable with background illness. MedPage Today asked Woodford if there might be a potential protective effect of malaria infection.

"That is a question much larger than me, and much larger than COVID," he said, adding that they found the rural areas in their study had higher rates of malaria than the urban sites, and the urban sites had higher rates of COVID than the rural sites.

"What that means, I'm not sure, but there's certainly a blunt association there," Woodford noted.

He cited another of hospitalized patients in Uganda, which found that patients with low versus high previous malaria exposure had higher risk of severe or critical COVID clinical presentation, even among patients with no comorbidities.

However, Woodford added there would be no way to tease out asymptomatic versus symptomatic infections without more accurate diagnostics, such as PCR testing, and a much larger sample size.

"You'd need a very large population to look at symptomatic versus asymptomatic in seropositive patients," he said. "Logistically, it's a very challenging study to put together."

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