The Oxford- AstraZeneca vaccine produced a strong immune response among the recipients, said a study by scientists at the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDRB) and the Institute of Epidemiology, Disease Control and Research (IEDCR).
Both the institutions said their findings were 'encouraging', raising hopes of strong protection for the age groups most at risk from the virus.
The vaccine recipients were aged between 40-73 years and from Dhaka city, with or without a history of Covid-19.
The ICDDRB and IEDCR scientists measured the levels of Covid-19 IgG antibodies in the blood of 120 participants before vaccination and then at one and two months after receiving their first dose of the vaccine through the government's vaccination campaign
It was found that one month after vaccination, 92 per cent of people who received the first dose of the vaccine have at least a high immune response, while 97 per cent of them have an even higher immune response two months after the first dose of vaccination.
The cut-off for a positive immunoglobulin G (IgG) antibody response to the spike protein of Covid-19 was set at 500 ng/ml using 355 serum samples collected before the onset of the pandemic.
Forty-six participants - with an average of 48 years of age and previous RT-PCR test confirmed Covid-19 infection history between April 2020 and January 2021 - were included in the analysis.
Participants who had a previous Covid-19 infection - one month or earlier before vaccination - showed a more robust response after receiving the first dose of the vaccine.
Fourfold higher antibody response was seen among these participants at two months after receiving the first dose of the vaccine.
Expert view: Too small to be conclusive
The task of a Covid-19 vaccine dose is to prepare the body to fight back against the virus "no matter how old someone is."
Dr Firdausi Qadri, a senior scientist at ICDDRB, said: "We now know that the Covishield vaccine triggers a robust immune response in Bangladeshi adults which is a piece of great news. However, we will continue working on evaluating the neutralising ability of the antibody and the T and B cell responses. We will also continue assessing the effectiveness of the vaccine in our setting."
Professor Tahmina Shirin, director of IEDCR, said: "Our analysis confirmed that the vaccine works and people should get it when their turn comes. However, everyone should continue wearing a face mask and maintain physical distancing along with personal hand hygiene."
However, former IEDCR director Dr AM Zakir Hussain said, "To ensure the validity of study findings, certain methodological aspects need to be stressed."
"The second issue is, the findings on antibody in itself does not mean protection unless it is shown to have neutralised the virus. The study is yet to explore this aspect," Dr Zakir added."Even the Oxford University researchers in at least three surveys did not find the protection level to be more than 80 per cent under any circumstances. This survey included 13 per cent of those who already developed an antibody from their infection before their vaccination. They cannot in anyways be included as samples for this study."
"The actual sample size is way below a requisite sample size to afford validity to the findings. IEDCR and ICDDRB in the past also revealed some study findings only to create commotion. I believe this study finding will also have to do a lot of explaining as too many questions will be raised again," the former IEDCR director added.
ICDDRB and IEDCR have started a large-scale study at 12 sites in eight divisions across Bangladesh that will involve about 6,300 participants who will complete their full course of vaccination.
Participants will be followed up for two years in a longitudinal design for assessing antibody responses after vaccination to determine the longevity of antibody response.
The Oxford University- AstraZeneca vaccine has been licensed for manufacturing under the brand Covishield by India's Serum Institute.