Herd immunity knocking on the door of Karachi: health expert

COVID-19 antibodies were absent from the blood of 17 per cent of the Karachiites who had contracted the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and tested positive for the viral infectious disease through the PCR (polymerase chain reaction) test, a top health expert of Pakistan has told The News.

“Sixty-eight of the 400 plasma donors who were infected with COVID-19 were tested through different methods but SARS-CoV-2 antibodies weren’t found in their blood,” said haematologist and bone marrow transplant surgeon Prof Dr Tahir Sultan Shamsi.

Dr Shamsi, who is leading the largest convalescent plasma therapy trial in the country, also believes that Pakistan, or Karachi at least, is knocking on the door of herd immunity, claiming that millions have already contracted COVID-19 asymptomatically and are immune to the infectious disease.

“Our research shows that 17 per cent of those who tested positive through the PCR test of various reputed labs didn’t develop COVID-19 antibodies. They were tested through latest methods, and after comprehensive testing it was concluded that despite getting infected with SARS-CoV-2, they couldn’t develop antibodies.”

He said this phenomenon is also being observed in other countries, as eight per cent of South Korea’s positive patients have not developed antibodies and four per cent of China’s, but the absence of antibodies in 17 per cent of Pakistan’s patients is quite a surprising discovery.

“But this doesn’t mean that these people aren’t immune against COVID-19 after getting infected with the coronavirus, as their bodies can provide immunity against it through memory T cells.”

However, Dr Shamsi maintained that it is unclear how long those who do have antibodies will remain immune to the antigen or the virus, saying that the presence of antibodies does not guarantee lifetime immunity.

He claimed that over 45 per cent, or even up to 50 per cent, of Karachi’s population has silently, or asymptomatically, contracted the infection, which he said is evident from the seroprevalence studies conducted at the National Institute of Blood Diseases in Karachi as well as by other health organisations.

He also claimed that within a few weeks Karachi might be able to achieve herd immunity, where transmission of the virus in the community would almost come to a halt.

He said that the dramatic decline in positive cases, reduced hospitalisation, the decreasing number of deaths and the antibody tests done on around 2,300 people from three different segments of society in Karachi since April indicate that 45 to 50 per cent of the city’s population has developed antibodies.

“Our research shows that 45 to 50 per cent of Karachiites have been infected asymptomatically, and within the next few weeks 65 to 70 per cent of the city’s population would’ve been infected, which is the minimum requirement for achieving herd immunity in the case of COVID-19.”

‘Not a big deal’

A top infectious diseases expert said that the absence of COVID-19 antibodies in the blood of positive patients is not a big deal, saying that the lack of antibodies does not mean a person has not cleared the infection.

“There are many arms and parts of the immune system. Antibodies are only one of them. So, the lack of antibodies doesn’t mean you haven’t cleared the infection. Immunity to SARS-CoV-2 is also complicated,” said Dr Faisal Mehmood of the Aga Khan University Hospital.

“You make many different types of antibodies, and not all of these are neutralising or able to block the virus. The kits used in the clinical lab are unable to differentiate between them and, hence, can’t be used to assess if a person is immune.”

And even if the antibodies are neutralising, he added, “we don’t know how long they last and what level needs to be maintained. Also, the place where the antibodies are — blood versus the nasal cavities — may be important”.

Dr Mehmood stressed that it is not as simple as only checking the antibodies in the blood to determine whether or not a person is immune.