MANILA, Philippines — After a local outbreak of the new coronavirus disease (COVID-19) was reported in March, the public has repeatedly called on the Department of Health (DOH) to conduct mass testing, especially since there were reported instances of patients turning out to be positive despite showing no symptoms.
For months, the DOH stayed firm that its testing strategy was to prioritize those who were symptomatic and those who were most at risk. But as the country’s testing capacity increased, the DOH said it would cover even asymptomatic persons, if they had visited other countries or an area where there is community transmission of the virus or was exposed to a confirmed case.
But what does the current available evidence really say about asymptomatic transmission? Is it really possible to catch the infection from someone who does not show any symptom related to COVID-19?
In April, the World Health Organization (WHO) said in a report that while “there has been no documented asymptomatic transmission,” it was possible that it could still occur.
New evidence
“There are few reports of laboratory-confirmed cases who are truly asymptomatic. Asymptomatic cases have been reported as part of contact tracing efforts in some countries,” the WHO said.
But as scientists better understand the disease, which originated in Wuhan, capital of China’s central Hubei province, Health Undersecretary Maria Rosario Vergeire on Thursday said there was emerging evidence showing instances when the virus could be spread by “presymptomatic” individuals, or those whose symptoms have yet to manifest.
“So experts had been saying on these recent [pieces of] evidence that presymptomatic transmission really occurs, and usually have the highest viral load, one to three days before the onset of symptoms,” Vergeire said.
“Because of evolving evidence, we found out that those asymptomatic [cases] are actually presymptomatic,” she added.
Vergeire explained that positive cases are often thought to be asymptomatic because the patients themselves may have had “very mild symptoms which they didn’t notice.” These may include a slight fever, headache or sore throat.
“Sometimes, it’s unnoticeable. Sometimes people just ignore the slight sore throat or headache, which may already be the presymptomatic, or mild symptoms of the symptomatic stage of COVID-19,” she said.
‘Most efficient’ transmitter
In the country, nearly all of the confirmed COVID-19 cases are mild or reported to be asymptomatic. In the latest data it released on Thursday, the DOH said 94 percent of the more than 15,000 active cases were mild, while 5 percent were listed as asymptomatic.
Dr. Anna Ong-Lim, the immediate past president of the Pediatric Infectious Disease Society of the Philippines, stressed that the public should remember that those with symptoms remain to be the “most efficient transmitter” of SARS-CoV-2, the virus that causes COVID-19.
She noted that based on studies, the rate of transmission from someone who is presymptomatic is “anywhere between 6 to 12 percent.”
“Yes [the possibility] is there, but it’s not as much infection as we could expect from someone who is already sick. We need to understand that not all who test positive are infectious. There is a difference between positivity and infectivity,” Lim said.
In a virtual press briefing, Dr. Maria Van Kerkhove, technical lead of WHO’s Health Emergencies Programme, said that a person with mild infection can remain positive for the virus for up to three weeks. Those with severe symptoms test positive under the polymerase chain reaction (PCR) test “for much longer,” the upper limit of which is yet to be determined.
“From what we know is that individuals who are from the mild end of the spectrum, from the time of symptom onset, the virus can be isolated from eight to nine days. The 10th day, [researchers] aren’t able to isolate the virus. So that gives us some clues about when a person may be more infectious and may be able to transmit to others,” Kerkhove said.
“We are also aware that asymptomatic cases can also be PCR positive for up to a week or so but we need more data to be able to characterize that well,” she added.
Recovered but still positive
According to Lim, because PCR tests are so reliable and highly sensitive, it is possible that someone will still test positive even if he is no longer infectious.
“There are patients who have already recovered but are still positive. This doesn’t mean that they are still infectious. [What is being picked up are] the fragments of the virus that are no longer infectious,” she said.
Vergeire noted that even with this development, what is crucial is to strengthen the current health measures being implemented—from surveillance to contact tracing and isolation of suspect cases.
“What is key is we are always able to isolate [cases] so that we can minimize the effect if ever [more of this] presymptomatic transmission occurs. And most importantly, of course, is the compliance to our minimum health standards,” she said.
The DOH on Saturday reported 714 new cases, pushing the national total to 21,340. Of the new cases, 350 were tested positive within the last three days—171 from Central Visayas and 104 from the National Capital Region.
The number of patients who had recovered climbed to 4,441 with the recovery of 111 more. The death toll rose to 994 after seven patients succumbed to the disease.