Studies from around the world published in the past weeks are revealing a fresh challenge in the fight against Covid-19, just as researchers are discovering the virus could be spreading undetected at a higher rate than initially known.

For South African experts, the studies from as far afield as Iceland, as well as California and Texas in the US, are guiding a path to this new battlefront of asymptomatic Covid-19 infections – but it remains to be seen how these cases could affect the spread of the disease across populations.

Until recently, the discussion around under-detection of cases centred on those who may have slipped through the net and remained unknown – which is generally what is accepted did happen, according to one of the National Institute for Communicable Diseases’ (NCID) top epidemiologists, Professor Cheryl Cohen.

But the studies are now showing early proof that another type of spreader, those who could be passing the virus on unknowingly – as they show no visible symptoms of the virus, or display symptoms not immediately associated with Covid-19 – are in fact contributing to the spread, confirming early assumptions by researchers.

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’s an important piece of information for us to understand how important the so-called under-detection is,” she said.

This week, Kingsway Hospital in KwaZulu-Natal was forced to shut down by halting new admissions, after at least six staff members contracted Covid-19 from a man who was admitted showing only the signs of a stroke.

Screening procedures in place at the hospital led to him being detected as a Covid-19 patient. But, he had no contact with high-risk individuals and had not travelled internationally. It remains unclear how he became infected.

As of Saturday evening, 52 people had died in the country from 3 034 confirmed infections. Laboratories around the country also passed a landmark 100 000 tests done this week.

An early travel ban instituted on 16 March by President Cyril Ramaphosa has been credited with slowing infections across the country, and the effect of the national lockdown remains to be seen.

As of this week, cases have again ticked up to an average of around 100 new cases a week, which had initially dropped to between 50 and 70 around the same time lockdown was instituted – an effect of measures put in place two weeks before, such as the travel ban.  

Higher spread

One study conducted in the California county of Santa Clara, which includes the city of San Jose, found the number of undetected cases of Covid-19 could be as high as 50-85 times more for every confirmed case. This study used blood tests to look for antibodies to the virus.

Cohen said, in the initial stages, the suggestion from China was that asymptomatic cases were not very common, but the mathematical models were not making sense, as the disease was behaving in a way that implied there should be asymptomatic infections.

Data coming out now was suggesting the asymptomatic cases were in fact more common than initially thought.

“One of the reasons they didn’t come out sooner is the common or standard way of measuring this is through serological or seroprevalance surveys… now the problem with that is the tests were not particularly sensitive and specific,” she said. 

“If we had those tests, studies would have been done in China and that question would have been answered.”

Serological surveys focus on testing blood samples of a population sample to search for, in this case, antibodies. The presence of antibodies to Covid-19 would mean the person had already had the virus, which would reveal the level of asymptomatic cases.

Rapid testing kits under development, which the National Health Laboratory Service hopes to roll out before the end of the month, would do exactly this in a large proportion of the population, providing a valuable detection and data collection tool.

A study by a team from the University of Texas in Austin found a 50% likelihood that if just one case of Covid-19 was confirmed, a sustained, undetected outbreak – an epidemic – was already happening.

In Iceland, a study was conducted on random citizens, mostly from the capital city of Reykjavik, which not only showed the benefit of aggressive and early screening and testing, but that around 50% of people tested were asymptomatic.

“What all of these studies are suggesting is that actually asymptomatic infections are much more prevalent than we thought,” Cohen explained.

Significantly, South Africa has embarked on an aggressive programme of screening and testing, with healthcare workers going door to door in areas identified as possible hot spots, which Cohen says was recognition of the under-detection that may be occurring.

Health Minister Zweli Mkhize said on Saturday night during a virtual press conference that around 900 000 people had been screened for Covid-19 symptoms, and roughly 11 000 of them referred for testing. 

Undetected

“There are two levels of under-detection. There is a level of under-detection where we understand we have symptomatic people who have fever and cough, they may be mild, they may be more severe, they may be in hospital, and we accept that we may not be catching and testing all of them,” Cohen sad.

That number could be as high as none or 10 for confirmed cases, some studies have shown.

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“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 degree that is. It is based on probabilities, it is likely that when we had that huge wave of importation from overseas, there would have been additional importations that would have gone undetected and once those importations spread to additional people, it would have been very hard to find them in the ocean of respiratory disease that we find ourselves in as we go into the winter season.”

READ: NICD’s top epidemiologist: How the Covid-19 pandemic is evolving and why we should be concerned

Cohen said the second issue around under-detection was the issue of asymptomatic cases.

“It is an emerging area of research, only in the last week or two a whole host of studies have started coming out, all pointing to the same thing, the same idea that there is likely a large pool of cases, I don’t know how large, but some people are saying that up to 70% of cases could be asymptomatic or mildly symptomatic,” Cohen said.

“That’s something that is going to have to be considered by mathematical modellers in SA and around the world, but it’s quite new, the evidence for that has only come out quite recently and that will, I think, 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 then is how impactful are they on a population level? That’s the difficult part.”