Although its study suggests around 71 percent of Dhaka city dwellers have already gained antibodies for coronavirus, an icddr,b scientist says herd immunity threshold is still out of reach in Bangladesh's capital, let alone the whole country to reach it, reports UNB.
Dr Rubhana Raqib, a senior scientist at the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b), also says it may not be possible to attain the long-term herd immunity for Covid-19 as long as the virus continues to mutate as she thinks highly contagious new variants can break people's immune protection gained either from the previous infections or vaccination.In an interview with UNB, she also said it is generally assumed that Covid-19 may remain active like influenza and other flues for a long time and it is quite possible that people will need to receive the vaccine at a regular interval until the virus loses its mutation or virulence capability.
icddr,b conducted a study titled "Driving Factors of Covid-19 in Slums and Non-Slum Areas of Dhaka and Chittagong," between October 2020 and February 2021 to evaluate the extent of the spread of the virus in the slum and non-slum communities of the two cities.
As per the findings of the study unveiled on June 22, Covid-19 antibody developed in 71 per cent people in Dhaka and 55 per in Chattogram while the overall 68 per cent of people studied had the coronavirus antibody developed in their blood. Dr Rubhana Raqib was the principal investigator of the study.
Herd immunity is a concept based on the body's immune resistance to the spread of a deadly disease (bacterial or viral infection) and it can be obtained in two ways -- naturally through infections of the majority of the population and artificially through vaccinating around 80-90 percent of the population of a country.
"No one, including the WHO, still can surely say whether herd immunity for the Covid-19 is possible. We know herd immunity for Measles, Polio, cholera and other diseases, but no one knows about it regarding the Covid," said Dr Rubhana.
She said it was a primary assumption that if around 80 percent of the population gains antibodies, then the herd immunity for Covid can be attained. "But new variants of Covid-19 virus are emerging with many mutations. So, it's still difficult to say how the antibodies will respond to the new variants.""For example, Israel has vaccinated almost 95 percent of its population with the Pfizer vaccine. But people in that country are now being infected with the Delta variant. That means achieving herd immunity regarding Covid is uncertain and difficult. So, we can't say the population in Dhaka is going to attain herd immunity," the scientist observed.
Covid vaccine may require every year
Rubhana said it may not be possible now to get herd immunity against Covid-19. "But it can be possible to protect people by boosting their immunity through administering vaccines from time to time as well as through upgrading the vaccines in line with emerging variants.
Like influenza and other flu vaccines, the scientist said it may require yearly vaccination for Covid. "The developed countries, including the USA, switch the flu vaccine strains every year evaluating the change in characteristics of those viruses. Most international scientists and WHO are also saying that it may require receiving Covid vaccines every year."
She said immunity from vaccines will give some protection against the new variants. "Virus has many antigens, but the vaccines are developed targeting some specific antigens for creating antibodies against those. When the virus changes its character, the antibody can't identify and fight the new antigens, but it can neutralise the old antigens and thus the antibody can reduce the severity of the disease."
Immunity can wear off
Rubhana said the antibody developed through Covid infection may not last long. "Research has shown that the antibody lasts for 3-4 months in some people while it lasts around 9-10 months in others. As the antibody is reduced after a few months, it can be boosted again with vaccination."
She said the human body contains B cells and T cells to protect the body from foreign invasion by viruses or bacteria. "These are the major cellular components of the specific immune response."
The scientist said when people get infections or are vaccinated, B cells in their body produce antibodies against the virus. "Once the infections subside, the B cells carrying the "memory" hide in the bone marrow. When these people encounter the virus again in the future, the B cells come out of their hiding place, multiply into thousands and millions of cells and produce huge antibodies to neutralise the virus."
Similarly, she said, "memory T cells" also remain hidden and when the virus attacks next time, they get stimulated with the exposure of the virus and kill the infected cells and the virus. "But it's still being investigated whether the B cells and T cells can properly work in neutralising or killing the coronavirus when it changes its characters through mutations," she observed.
No alternative to vaccine
Asked which antibody, natural one or vaccine-generated one, is stronger, Rubhana said, "Vaccines train our immune system to fight against future infections with the new coronavirus. But there's an interesting thing regarding the Covid that if any person having natural antibodies from a Covid infection receives a vaccine, it significantly boosts his/her immunity. Research has shown that the response of antibodies from vaccines is higher than the antibody from infections."
She said some vaccines, such as Pfizer and Moderna, have been developed considering some components of the virus while Sinopharm developed it considering the whole inactivated virus. "So, these two types of vaccines may give different levels of protection."
She also said those who receive vaccines may not suffer from severe infection if they get infected with the virus afterwards. "So, there's no alternative to vaccination to get back to normalcy."
Rubhana said serosurveillance study should be conducted across the country to know the extent of the spread of the disease and the level of antibodies among the vast population.
"It's also necessary to know how long the antibodies persist among the different age groups of people and how the antibodies work against the new variants like Delta one," she added.