Can those who test negative still spread COVID-19?
Asymptomatic carriers may be more than previously imagined, and they may be responsible for ‘viral shedding’ and disease transmission.
There is mounting evidence for some unsettling COVID-19 findings; asymptomatic carriers may be more than previously imagined, and they may be responsible for ‘viral shedding’ and disease transmission.
What makes COVID-19 especially dangerous from a public health perspective is that it is quite transmissible (an infected person can infect 2 to 3 people on average) and that, unlike SARS and MERS, there are many exhibiting mild or no symptoms. While COVID-19 is less deadly than SARS (around 2% mortality rate compared to 10%), less sick people seem to be spreading the infection without realizing, hampering containment efforts.
Data from quarantine facilities from Wuhan have suggested that between 5 to 10% people considered ‘recovered’ tested positive again on retesting. While this could be a result of faulty testing (some Chinese academicians believe that the tests used in China were only up to 50% accurate; the whistleblowing Dr Li Wenliang actually tested negative several times before being accurately diagnosed), it further raises concerns about unchecked community transmission.
It is less likely that the same person can get infected twice in such a short period, according to virologists. However, this does not mean that people are no longer contagious.
Can those who test negative continue to transmit the disease?
A study published in the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine last week presented data from 16 patients who had tested negative according to two PCR tests. It found that at least half the patients could be shedding the virus even after testing negative - throat swabs on alternate days showed remnants of the virus in the body.
The patients remained contagious between one to eight days after testing negative. The median age of patients was 35.5 years, and the average duration of symptoms was 8 days.
The researchers also suggested that more severe infections could lead to longer shedding times, but more research is needed to study this claim.
While the study was small, it adds to the evidence of asymptomatic spreaders. The Diamond Princess, the cruise ship that was docked outside Japan because of positive cases, surprised experts worldwide with the rapid spread of infections despite cautionary measures and repeated testing. About 700 out of 3700 passengers ended up infected; studies have shown that around 18% of them showed no symptoms. These asymptomatic cases probably propelled infections in the vessel.
What are the implications of the findings?
The researchers from the Thoracic Society recommended that those who have mild symptoms should self-isolate up to two weeks after symptoms pass. Given that the virus may still be shedding, it is best to practice social distancing to limit transmission.
This is supported by a previous study in the New England Journal of Medicine that showed that asymptomatic people shed a considerable amount of virus. Further, children have so far been shown to not suffer grievously from the disease, but they can also be transmitting the disease.
For more tips, read our article on How COVID-19 Spreads.
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