The Oxford-AstraZeneca vaccine's first shot starts its effect three weeks after it is administered and remains effective up to three months, according to a latest study.
Experts said there might be misconception among some people that Covid-19 vaccines provide immediate protection.
They also said that there was also no evidence that any of the current Covid-19 vaccines could completely stop people from being infected, advising people to continue to wear masks and maintain other health guidelines laid out by the World Health Organization.
Bangladesh has been administering the Oxford vaccine across the country among people aged 40 years and above. So far, around 21 lakh people have received their first dose.
The issue of how much and from when the vaccine will provide immunity surfaced after a vaccine recipient tested positive for Covid-19 seven days after receiving the first jab in Dhaka on February 10.
Fifty-seven-year-old SM Golam Kibria, in a Facebook status on February 19, wrote: "I have become infected on the sixth day… So do not [make the] mistake [of] thinking your are safe just by taking vaccines."
He also shared a copy of Covid-19 positive RT-PCR test result from the Institute of Epidemiology, Disease Control and Research (IEDCR). The IEDCR collected his samples on February 17.
According to the Oxford Covid-19 vaccine website, the efficacy is 76 percent between 22 days and 90 days after the first dose.
"After inoculation, the Oxford's Covid-19 vaccine takes time to produce antibodies. May be this person [Kibria] was exposed to the virus days earlier. It is normal," Prof Tahmina Shirin, director of the IEDCR, told The Daily Star on Friday.
"The person who gets vaccine doses is equally vulnerable to Covid-19 infections like the one who did not get it, until the creation of enough antibodies," she added.
"Irrespective of whether you are vaccinated or not, everyone should wear masks and maintain physical distancing at least till the end of this year," said Tahmina.
Khondoker Mehedi Akram, senior researcher at the University of Sheffield, said the efficacy of the Oxford vaccine's first dose reaches its peak after 21 days.
"But before that, they are unprotected like those who are not vaccinated. They must remain careful during that period," he told The Daily Star yesterday.
On www.gavi.org, health experts also explained some other reasons why people should hold on to their masks.
No vaccine is 100 percent effective.
They are usually slightly less effective in the real world compared to clinical trials. The effectiveness of Covid-19 vaccines also varies depending on how efficiently they are handled. It may not prevent one from spreading the virus, said experts.
Researchers are yet to figure out whether Covid-19 vaccines prevent transmission.
The preliminary evidence of a study suggested the Oxford-AstraZeneca vaccine could slow the transmission of the virus. But some epidemiologists said it was too early to say this, according to the Gavi.
They said people who have had one or two doses of the Oxford-AstraZeneca vaccine should not lower their guard in terms of physical distancing, wearing masks or hand hygiene.