New surveys carried out by the Nigeria Centre for Disease Control (NCDC) suggest that Covid-19 infections could be considerably higher than the number of confirmed cases, with more than a fifth of Nigerians in some states testing positive for antibodies.
Millions of Nigerians could possibly be infected by the novel coronavirus, in stark contrast to the country's 152,000 confirmed cases, according to analysis of the study, which was carried out between September and October 2020.
Blood samples from more than 10,000 people were collected across four states and then tested for the presence of Covid-19 antibodies.
Antibodies were present in 23% of people in heavily populated Lagos State and south-central Enugu State. Nineteen percent of people tested positive in Nasarawa State, in Nigeria's Middle Belt, and 9% in northern Gombe State.
"I am surprised that it is as high as 20%," said Adeniyi Fagbamigbe, an expert in medical statistics at the University of Ibadan. "This is shocking and it's dangerous for the control of the pandemic," he added.
The results also showed that more men tested positive compared to women, and urban areas had a higher rate of infections in contrast to rural areas.
NCDC's study selected volunteers from representative households in the four states, and noted variations across states matching trends in the country's national surveillance system.
Medical statistician Fagbamigbe told RFI that he agrees with the NCDC that the study underlines the significant number of people who are asymptomatic or only display mild cases of the virus.
"A lot of Nigerians are asymptotic, I agree with that, I believe that the virus is there, but it is not as dramatic as we find in the Western countries," he said, pointing to the high death toll in many European countries.
The true number of those infected could be higher still given the nature of antibody testing for Covid-19. Some people may not develop antibodies and people tested outside of a three week window, in which antibodies are present, may test negative.
At first glance, the surveys could suggest that millions of Nigerians may already be carrying the virus. Lagos State is thought to have a population of some 25m people, and the study could be extrapolated to suggest at least 5m already infected.
However, results from such a study are not always indicative of the situation across an entire country.
"I cannot say directly how this reads from a 10,000 subset of the population, inferred on the whole population," according to Fagbamigbe, an affiliate of the African Academy of Sciences, who has published research on Covid-19.
"Was there randomisation as far as the selection of respondents was concerned, were there specific areas, we need to know these details," he said, describing concerns about using such a study to estimate the impact on the whole of Nigeria.
Nigeria, Africa's most populous country, has more than 211m people, according to a 2019 estimate by the UN.
Despite Nigeria possibly having drastically more infections than previously thought, the health authorities warn many more people could still be at risk.
"It is very important that Nigerians continue to adhere to public health and social measures including regular handwashing, proper use of face masks and physical distancing," the NCDC said in a statement.
Nigeria has registered over 1,800 deaths from Covid-19, according to official statistics.