Asymptomatic people are more prone to reinfection with Covid-19 as they lose protective antibodies quicker than those who exhibit symptoms, according to a new research.
Health experts say it is now necessary to do a research to know why levels of antibodies in those people fade rapidly.
Genome sequencing can confirm whether the second-time infected people were attacked by the same virus or another type, they add.
Antibodies are a key part of immune defences and stop the virus from getting inside the body's cells.
Prof Samir Kumar Saha, executive director of Child Health Research Foundation, told The Business Standard that 29 people have so far been re-infected with Covid-19 worldwide. There are reports of second- and third-time infections in Bangladesh but not proven yet.
"We can confirm whether they are re-infected with the same virus or new ones through genome sequencing. Nothing can be said before a study is done in this regard," he added.
The findings by Imperial College London and Ipsos Mori, a global market research firm, disclosed on Tuesday showed the number of people with antibodies fell by 26.5% over the approximate three-month period.
They also suggest the loss of antibodies was slower in 18-24-year-olds compared to those aged 75 and above.
When contacted, Professor Nazrul Islam, noted virologist and former vice-chancellor of Bangabandhu Sheikh Mujib Medical University, told The Business Standard that antibodies in the Covid-19 sufferers normally stay approximately three to four months. But the prevalence of antibodies in asymptomatic cases is very low. So, they are at greater risk of reinfection and their health conditions might turn critical.
Lots of things about coronavirus are still unknown to scientists. Therefore, the patients who have recovered from Covid-19 should strictly follow health guidelines, he added.
In Bangladesh, 401,586 people have so far been infected with coronavirus. Of them, 318,123 have recovered.
Some 82% of Covid-19 patients in the country are asymptomatic, according to a recent study conducted by the Institute of Epidemiology, Disease Control And Research and the International Centre for Diarrhoeal Disease Research, Bangladesh.
Patients recovering from Covid-19 have been reported to be infected for the second and even third time in the country, according to several hospital sources.
Dr Uttam Barua, director at Shaheed Suhrawardy Medical College and Hospital, said most of the 180 health workers who contracted Covid-19 at his hospital for the first time were asymptomatic. But those who were infected for the second time exhibited symptoms. Even a doctor who caught the virus for the third time was symptomatic.
"We collected blood samples from the re-infected health workers. We, along with the Child Health Research Foundation, will conduct a research on why the health workers caught the virus multiple times."
The Imperial College study found immunity appears to be fading and there is a risk of catching the virus multiple times.
"It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts," said Paul Elliott of Imperial's School of Public Health.
Helen Ward, one of the lead authors, said, "This very large study has shown that the proportion of people with detectable antibodies is falling over time."
"We do not yet know whether this will leave these people at risk of reinfection with the virus that causes Covid-19, but it is essential that everyone continues to follow guidance to reduce the risk to themselves and others," he added.