Is Covid-19 ‘herd immunity’ really working in Pakistan?

Since coronavirus became a global health emergency, one that would go on to kill a million people, overwhelming hospitals and leading to widespread job loss, doctors and researchers have wondered if herd immunity will come into play to contain it. 

Herd immunity is described as a situation wherein a large part of the population has already been infected with a virus, making its further spread difficult. 

In recent weeks, multiple surveys have surfaced suggesting that a few cities, if not entire countries, might be showing signs of such widespread immunity to Covid-19, the disease caused by the novel coronavirus. 

Researchers primarily look for antibodies, the molecular fighting mechanism in our bodies, to see if a patient has been infected. 

One of the most interesting - and controversial - findings has come out of Karachi, Pakistan’s largest city. Epidemiologists at a leading institute there took samples from 1,675 people between May and July to figure out why the pandemic was tapering off in the country. 

“The intensity of infections in the developed world was very different from ours. Obviously, we were having infections but we were not having that high case fatality rate and high mortality,” says Samreen Zaidi, who co-authored the report. 

After the initial coronavirus cases were reported, there were concerns that the health infrastructure in cash-strapped Pakistan - a country of 220 million people which has only 2,000 ventilators - won’t be able to cope with the pandemic. 

But after a spike, during which graveyards were quickly filled and Facebook pages became flooded with pleas for plasma donations, the death rate went into decline. 

Zaidi’s team found that around 36 percent of the surveyed people had developed immunity, which basically means the human body has become equipped to fight a pathogen. In one subset of the sample consisting of employees of a pharmaceutical company, they found seroprevalence, which is the number of people infected, to be around 50 percent. 

“We were very surprised when the results came in and we went back to interrogate them further. These people were basically asymptomatic or had mild symptoms such as a day-long fever. 

“A coronavirus infection didn’t even cross their mind. But they had positive antibodies,” says Zaidi, who works for Karachi-based National Institute of Blood Diseases (NIBD). 

Authors of that study now believe that 60 percent of the people might have achieved the immunity level based on fresh data that they have collected. 

The great unknowns 

A similar study carried out in the Brazilian city of Manaus, where the number of people dying because of Covid-19 dropped after hitting a peak in May, also says that its residents might have reached herd immunity. 

Around 60 percent of the surveyed people showed signs of antibodies in a recent survey - the results of which were published on the medRxiv website. 

Experts have yet to agree on what size a given population has to become immune to go on to qualify for herd immunity, but it is generally understood to be between 60 and 70 percent. Vaccinations against polio, measles and other infections play a similar role in building up immunity. 

Both the Karachi and Manaus surveys face a handicap: their sample size, which runs into hundreds of voluntary blood donors, is too small to reflect the reality of a country or even a large city. 

“We have been open about the limitations of our study and have added a disclaimer that our small sample size cannot represent the entire 20.5 million to 30 million population of Karachi,” says Zaidi. 

The authors of the Pakistan study have asked the government to carry out a survey on a national scale to determine if herd immunity is really happening. 

In any case, some leading experts have been sceptical about the whole idea. Developing immunity via natural selection means authorities are willing to let the pandemic run wild, killing tens of thousands, before it stops spreading. 

When viewed in the context of sheer numbers, the concept of herd immunity appears terrifying.

For example, South Korea is reporting 50 new Covid-19 cases a day. “If they were to hold on for another thousand days, which is three years, they would have 50,000 cases, which is 0.1% of their population," Jeffrey Shaman, a professor of environmental health sciences at Columbia University, told NPR. 

Similarly in the United States, a country of more than 320 million, it would take months and hundreds of thousands of casualties before the required threshold of 60 percent is reached. 

For poor and developing countries lacking the resources to deal with the pandemic, such a strategy could be devastating. 

The surveys and studies on herd immunity should be taken with a pinch of salt, especially as there are concerns about the accuracy of antibody tests, says Dr Jeremy Rossman, an honorary senior lecturer on virology at the University of Kent. 

"The studies that have gone through the peer review process tend to report less than 10 percent seropositivity rates (occasionally up to 15 percent in dense urban areas), whereas many preliminary reports from various countries have reported much higher rates," he tells TRT World in emailed comments. 

There are also questions about how long the immunity lasts. Multiple reports have shown that immunity can disappear within a few months. 

"We have already seen documented cases of reinfection occurring and we know that in many people antibody levels decline rapidly after infection. This suggests that herd immunity may not even be possible for SARS-CoV-2, but we don’t know for certain yet," says Rossman. 

In Pakistan, Zaidi says, doctors have observed a milder form of Covid-19. 

“We are not sure why that is. But it could be because of cross-infections as people have already been infected with other diseases and acquired immunity. The prevalence of dengue fever is also being touted as another reason,” she says.