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Studies from around the world published in the past few weeks reveal a new challenge in the fight against Covid-19, just as researchers are discovering that the virus could spread undetected at a higher rate than initially known.
For South African experts, studies from places as far away as Iceland, as well as California and Texas in the United States, are guiding a path to this new battlefront of asymptomatic Covid-19 infections, but how these cases remain to be seen. could affect the spread of the disease through populations.
Until recently, the discussion of case underdetection focused on those who may have slipped online and remained unknown, which is generally accepted, according to one of the leading epidemiologists at the National Institute of Communicable Diseases (NCID). ), Professor Cheryl Cohen.
But studies now show early evidence that another type of diffuser, those that could unknowingly transmit the virus, since they show no visible symptoms of the virus or show symptoms not immediately associated with Covid-19, are in fact contributing to the spread, confirming the researchers’ first assumptions.
Cohen said a question mark hangs over the likelihood of these asymptomatic infections, which some studies believe could be as high as 70% of infections, transmitting the virus, which is likely much lower than severe cases.
“That is the great thing that is not really known, and it is important information for us to understand how important so-called underdetection is,” he said.
This week, Kingsway Hospital in KwaZulu-Natal was forced to close by suspending new admissions, after at least six staff members hired Covid-19 from a man who entered showing only signs of a stroke.
Detection procedures at the hospital led him to be detected as a Covid-19 patient. But he had no contact with high-risk people and had not traveled internationally. It is unclear how he became infected.
As of Saturday night, 52 people had died in the country from 3,034 confirmed infections. Labs across the country also passed 100,000 tests this week.
An early travel ban initiated on March 16 by President Cyril Ramaphosa has been credited for declining infections across the country, and the effect of the national blockade remains to be seen.
As of this week, cases have returned to an average of around 100 new cases per week, which initially dropped to between 50 and 70 at approximately the same time that closure was instituted, an effect of the measures implemented two weeks before, such as a travel ban.
Greater spread
A study conducted in California’s Santa Clara County, which includes the city of San José, found that the number of undetected cases of Covid-19 could be as high as 50-85 times more for each confirmed case. This study used blood tests to look for antibodies against the virus.
Cohen said that, in the initial stages, China’s suggestion was that asymptomatic cases were not very common, but that mathematical models made no sense, as the disease behaved in a way that implied that there should be asymptomatic infections.
The data coming out now suggested that asymptomatic cases were, in fact, more common than initially thought.
“One of the reasons they didn’t come out earlier is the common or standard way to measure this is through serological or seroprevalence surveys … now the problem is that the tests were not particularly sensitive and specific,” he said.
“If we had those tests, the studies would have been done in China and that question would have been answered.”
Serological surveys focus on analyzing blood samples from a population sample to look, in this case, for antibodies. The presence of antibodies against Covid-19 would mean that the person had already had the virus, which would reveal the level of asymptomatic cases.
Rapid test kits under development, which the National Health Laboratory Service hopes to implement before the end of the month, would do exactly this for a large proportion of the population, providing a valuable tool for detection and data collection.
A study by a team at the University of Texas at Austin found a 50% chance that if only one case of Covid-19 was confirmed, a sustained and undetected outbreak, an epidemic, was already happening.
In Iceland, a study was conducted on random citizens, mainly from the capital city of Reykjavik, which not only showed the benefit of aggressive and early detection and testing, but around 50% of the people tested were asymptomatic.
“What all these studies suggest is that asymptomatic infections are actually much more frequent than we thought,” Cohen explained.
Significantly, South Africa has embarked on an aggressive screening and testing program, with health workers going door-to-door in areas identified as potential hot spots, which Cohen says is a recognition of underdetection that may be occurring.
Health Minister Zweli Mkhize said Saturday night during a virtual press conference that around 900,000 people had been screened for Covid-19 symptoms, and about 11,000 of them had been referred for testing.
Not detected
“There are two levels of underdetection. There is a level of underdetection where we understand that we have symptomatic people who have fever and cough, they may be mild, they may be more serious, they may be in the hospital and we accept that we may not be catching and testing them everyone, “said Cohen sadly.
That number could be as high as none or 10 for confirmed cases, some studies have shown.
NICD Professor Cheryl Cohen and Health Minister Zweli Mkhize addressed the media. (Jan Gerber)
“I would say that it is generally accepted that we are missing some of the cases, but it is very difficult to quantify to what extent it is. It is based on the probabilities, it is probable that when we had that great wave of importation from abroad, there would have been imports additional that would not have been detected and once those imports spread to additional people, it would have been very difficult to find them in the ocean of respiratory illnesses that we encountered as we progressed through the winter season. “
READ: The NICD’s Leading Epidemiologist: How the Covid-19 Pandemic Is Evolving and Why We Should Worry
Cohen said the second problem related to underdetection was that of asymptomatic cases.
“It is an emerging area of research, only in the last week or two a large number of studies have started to be published, all pointing to the same thing, the same idea that there is likely to be a large group of cases, I don’t know how big, but some people say that up to 70% of cases could be asymptomatic or mildly symptomatic, “said Cohen.
“That is something that mathematical modelers will have to consider in SA and around the world, but it is quite new, the evidence for that has only come out very recently and I think that will be the emerging story.”
She said there was evidence that asymptomatic people can transmit Covid-19, but they would likely transmit at a lower rate than symptomatic cases.
“The question is, how impactful are they at the population level? That’s the hard part.”
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