The revelation on Monday about prevalence of antibody against Covid-19 in most Dhaka slum residents indicates that the impoverished communities were widely infected and then recovered but they mostly remained outside the purview of polymerase chain reaction (PCR) tests.
Experts, however, say a wider research is needed to come to a conclusion about the development of antibodies at a rate as high as 74% in the slum population where infection rate was just 5.6%.
The Institute of Epidemiology, Disease Control and Research (IEDCR), in a survey conducted between 14 April and 5 July, found 74% of the slum population have developed antibodies against the virus.
The data were extracted from antibody tests in 125 individuals selected randomly from 960 households in eight slums of the capital city.
Dr ASM Alamgir, principal scientific officer of the IEDCR, said PCR tests in the slum people had been very limited, which is why the number of reported cases among them was very low.
"But they, being mostly young, have suffered and recovered," he said, adding that, yet, more research is required to make a conclusive remark about it.
Dr M Mushtuq Husain, adviser to the IEDCR, also suggested the slum data are not representative of the whole populace because the sample was too small.
During the same period, 45% of 692 individuals outside slums were also found to have antibodies. That means at least 45% of them had Covid-19 who eventually recovered.
There is a possibility of higher infections because not all infected people develop antibodies, Mushtuq added.
The daily infection rate from RT-PCR tests across the country within the timeframe hovered between 10% and 23%.
The 2.5-month cumulative data of the IEDCR point to a gradual decline in infections and a growth in people's immunity as bodies generate antibodies against the virus, according to Prof Nazrul Islam, a noted virologist.
"As people recover, the infection rate comes down but the antibody rate goes up," he said.
The data from the National Institute of Laboratory Medicine and Referral Centre since the end of July also corroborate his observation.
AKM Shamsuzzaman, director of the institute, said the PCR testing centre tested 63,272 individuals who needed a Covid-19 report before flying abroad from 20 July to 12 October. Only 951 of them tested positive for the virus.
One reason might be that many of them had already recovered from Covid-19 and developed antibodies. Antibody tests could help to understand that, he also said.
Meanwhile, the antibody rate in slums gives an indication of the herd immunity. In that case, these people are highly unlikely to suffer from a second wave of infection, Prof Nazrul said.
Over the next months, if the infection rate in the city population outside slums reaches over 70%, they will also become immune to the virus, he added.
Prof Nazrul, however, warned people may suffer in the upcoming winter because, generally, the resistance capacity of a human's respiratory tract system falters at that time.
There is another possibility that when a different virus will occupy the respiratory organs during winter, it will keep the novel coronavirus out from infecting people, he also said.
"But, no conclusion can be made until we spend one winter with the virus," Prof Nazrul added.