Kemri revises down number of Kenyans exposed to Covid-19

The number of Kenyans exposed to the novel coronavirus has been revised down to about 1.6 million from the initial estimate of 2.7 million following further analysis of antibody tests.

In a study carried out on samples of blood donated by Kenyans between April and June, researchers at the Kenya Medical Research Institute (Kemri) Wellcome Trust in Kilifi made the revision but said that while the new number of Kenyans who have developed antibodies against coronavirus is lower than initial estimates, exposure in the entire population could be higher as the 1.6 million only represents those aged 15 to 64.

The Ministry of Health has endorsed the study published six days ago in the Journal medRxiv, a preprint server for Health Sciences.

An initial analysis released early last month had showed 2.7 million Kenyans were exposed to the virus, based on antibody tests on 2,535 donated blood samples from across the country. This shows there could be a huge gap between confirmed cases and the number of people exposed to the coronavirus.

Antibodies are protective proteins produced by the immune system in response to or counteracting the presence of a foreign substance --known as an antigen-- such as bacteria and viruses.

The antibody test determines if a person has had an infection with the virus that causes Covid-19.

In the new study, the team of scientists led by Dr Sophie Uyoga, a researcher and immunologist at Kemri-Wellcome, analysed 3,098 samples of blood donated by Kenyans, 15 to 64 years between April 30 and June 16.

The analysis shows infection prevalence in Mombasa is at 9.3 percent, Nairobi 8.5 percent, and Kisumu 6.5 percent. The scientists argued that there is no evidence that recovered patients can be re-infected with Sars-CoV-2, the virus that causes Covid-19, and anecdotal reports of reinfection may be cases of drawn-out illness.

Working with an estimated country's population of 53 million, 57 percent of whom are aged 15-64, a seroprevalence of 5.2 percent would, therefore, suggest approximately 1.6 million infections in Kenya. Seroprevalence refers to the frequency of individuals with antibody to a particular virus in a population.

Tests on the samples showed 5.6 percent (174 out of 3,098) had Covid-19 antibodies. Covid-19 antibodies appear within one to three weeks after symptom onset and are weaker in people with fewer symptoms and those with no symptoms.

The proportion of Covid-19 cases that are asymptomatic varies by setting, in countries like Kenya, with a young population, it is likely to be high, the scientists noted.

“We estimate that one in 20 adults in Kenya had Sars-CoV-2 antibodies during the study period,” say the researchers.

Seroprevalence studies based on antibody surveys can be used to monitor the spread of a pandemic across the population and to inform mathematical models that predict its course.

Because blood donors are not representative of the Kenyan population, the researchers adjusted for the bias by standardisation against the age, sex, and regional distribution of the Kenyan population.

The adjusted national prevalence was fixed at 5.2 percent. Further, when the last blood samples were collected on May 30, the country had only 2,093 Covid-19 cases and 71 deaths reported through the national screening system. As of Monday, reported cases had risen to 22,597 and 382 deaths.

Compared to the population structure from the 2019 Kenya Population and Housing Census, the study participants were predominantly male, had more persons aged 25 to 34 and many were residents of coastal counties.

In addition, the 15- to 64-year-old category sampled in the study comprises only 57.1 (27,150,165) of the total population (47,564,296) of Kenya in 2019.

Ministry of Health's chief administrative secretaries Dr Rashid Aman and Dr Mercy Mwangangi, and the Director-General for Health Dr Patrick Amoth co-authored the study. 

Others are Kemri-Wellcome Trust Nairobi director Edwine Barasa, Kenya National Blood Transfusion Services boss Charles Rombo, and Kemri's Dr Isabella Oyier and George Warimwe.