‘Unexplained decline in COVID-19 cases’ in Pakistan puzzles health experts and policymakers

The presence of a kind of “non-specific immunity” that is unique to the Pakistani people due to multiple exposures to various vaccines, including BCG and Oral Polio Vaccine (OPV), high humidity in the current monsoon season and a low transmission rate in the peak summer season could be some of the factors slowing down the COVID-19 pandemic in Pakistan, health experts and officials told The News on Tuesday.

“There is no doubt that the percentage of COVID-19 positive cases, the number of hospitalised patients and deaths have been declining in Pakistan since the start of July 2020. As far as the reason behind this decrease in cases is concerned, it could mainly be due to the presence of a non-specific immunity that is unique to Pakistanis due to multiple exposure of various vaccines, highly humid weather and low transmission season of respiratory illnesses in the peak summer,” opined Dr Rana Muhammad Safdar, a leading epidemiologist and coordinator of the National Emergency Operation Centre (EOC), Islamabad. Several other leading infectious diseases’ experts and officials also conceded that the percentage of COVID-19 cases had been declining since the start of July 2020 in Pakistan, but they added that nobody knew the reasons behind the slowing down of the pandemic. They said this phenomenon was also being observed in regional countries Afghanistan, Bangladesh and Iran, except India where positive cases and the deaths were continuously on the rise

‘Non-specific immunity’

An expert of “emerging infectious diseases”, Dr Rana Muhammad Safdar, said the presence of “a non-specific, unique” immunity could be one of the reasons that COVID-19 positive cases had started declining cases in Pakistan.

He added that in addition to producing a specific immunity, all vaccines produced an innate, non-specific immunity in the human beings that prevented them from other pathogens also. 

“Our population has been exposed to so many vaccines in their lifetimes, including BCG (Bacillus Calmette–Guérin) vaccine, which is primarily used against tuberculosis and OPV, that they must have strong immune responses against several other pathogens, including the novel coronavirus. This non-specific immunity is playing an important role in slowing down the spread of this virus and reduced mortality, as compared to other countries of the world, in Pakistan.”

Dr Safdar maintained that a highly humid weather in the current monsoon season is also playing an important role in slowing down the spread of the coronavirus in Pakistan, saying that due to humidity, air becomes heavy and droplets containing the virus don’t go far when a carrier speaks, coughs or sneezes.

“And lastly, it is low transmission season for respiratory viruses and its transmission in the sunlight is very slow. It is still spreading but mostly in shaded places,” Dr Safdar said.

Reduced testing

Commenting on the reduced number of tests, he said that although the number of COVID-19 tests had decreased during the last two weeks, the percentage of positive cases had also decreased drastically, which showed that a smaller number of people were contracting the disease in Pakistan.

“At the peak of COVID-19 cases, 22-23 per cent people were testing positive in June 2020, but now the percentage of positive cases has dropped to 10 per cent or less. The decline in cases is also evident from the reduced hosptialisation and a lesser number of people requiring ventilators’ support,” he added.

Why tests fell?

Dr Rana said a lower number of tests in Pakistan were due to two major shifts: “first because exit test recovered patients is no more mandatory while no more mandatory testing of all international arrivals is being implemented”.

“The third contributory factor is the significant slowing down of the epidemic everywhere except Karachi, which is evident from decreased positivity along with case numbers, fewer suspects reporting for testing by themselves, a lesser load on hospitals, declining numbers of patients on oxygen and on ventilators, stable death counts, lesser daily healthcare infections and stabilisation of epidemic in Southeast Asia as a whole except India,” he added.

“The future, however, remains uncertain, as besides virus drop in new population settings, the upcoming Eid is a major risk. However, at least for the current wave, positive indications cannot be denied,” he further said.

But several other experts, when approached by The News, expressed their inability to answer as to why the pandemic was slowing down in Pakistan, saying no concrete scientific evidence of the role of innate, non-specific immunity, humidity or the weather was available yet.

Leading infectious diseases’ specialist at the Aga Khan University Hospital Dr Faisal Mehmood said the role of innate, non-specific immunity in slowing down COVID-19 was not known. “However, we do see a lot of sick people here as well. Research is ongoing on this.”

When asked if humidity slowed down viral transmission, he said: “Yes and no. Very dry air may help the droplet particles travel further in closed, likely ventilated settings. However, very high humidity may also increases viral spread.”

The prime minister’s focal person on COVID-19 and leading infectious diseases’ specialist, Dr Faisal Sultan, was of the opinion that the slowing down of COVID-19 cases due to the presence of non-specific immunity among the Pakistani people “is technically a possibility”, but no evidence for this idea was relevant in this case, “at least so far”, he added.

Responding to the possibility of the pandemic’s slowing down due to high humidity, he termed it “a conjecture, mostly”. “Actually 200 million people don’t live in a single container or space. Epidemics spread within social networks and local containers; they also slow down when the container gets saturated. This is a complicated one,” he added.

Dr Sultan opined that the decrease in COVID-19 cases was likely due to a mixture of more than one thing. “The adoption of masks and some NPIs helps in addition to some other factors perhaps. The flu/cold season is always a worry for respiratory viruses, but again this virus keeps surprising the world, so who knows,” he added.

Pathogen losing virulence?

Conceding that positive cases in Islamabad had dropped to eight per cent from 22 per cent in June 2020, Dr Muhammad Salman, a leading scientist and virologist at the National Institute of Health (NIH) Islamabad, said, “Nothing is known for sure how this novel coronavirus is behaving despite the fact that a lot of research has already been undertaken on it all around the world.”

“The decrease in the number of positive cases in Pakistan and other regional cases may be because the virus is losing its virulence. Sometimes when the virus transmits to so many people, it loses its virulence due to mutations, but again that needs to be researched. At the moment, we don’t have any specific answer to this question,” Dr Salman added.