There are gaps in knowledge about the contribution to the spread of SARS-CoV-2 virus, which causes Covid-19, by infected persons who have no symptoms (asymptomatic), infected persons but are yet to have symptoms (pre-symptomatic) and infected persons with mild symptoms.
Many experts believe that these persons, who are unnoticed by anyone, could be an important source of the spread of the infection.
The World Health Organization’s (WHO) definitions are:
• An asymptomatic laboratory-confirmed case is a person infected with Covid-19 who does not develop symptoms. Asymptomatic transmission refers to transmission of the virus from a person, who does not develop symptoms.
• The incubation period for Covid-19, which is the time between exposure to the virus (becoming infected) and symptom onset, is on average 5-6 days, however can be up to 14 days. During this period, also known as the “pre-symptomatic” period, some infected persons can be contagious. Therefore, transmission from a pre-symptomatic case can occur before symptom onset.
The first report of spread by an asymptomatic person was that of a 20-year Wuhan woman who spread the SARS-CoV-2 virus to five family members (JAMA. 2020; 323(14):1406-1407. doi: 10.1001/ jama.2020.2565).
An analysis of 262 confirmed Covid-19 cases hospitalised in Beijing by 10 February 2020 to determine the clinical and epidemiological characteristics in Beijing reported that 13, i.e. 5% were asymptomatic.
The Diamond Princess cruise ship was quarantined off Yokohama from 5-21 February 2020 when a passenger tested positive subsequent to disembarking in Hong Kong. Of the 3,063 tests done, 634 tested positive then. It was estimated that 17.9% of those who tested positive were asymptomatic with most of the infections acquired before or around the start of the quarantine. (EuroSurveill 2020; 25.doi:10.2807/ 1560-7917. ES.2020.25.10.2000180)
This overlapped with the 33.3% estimate from the data of Japanese citizens evacuated from Wuhan.
A study of 76 (93%) of 82 residents in a Long-Term Care Skilled Nursing Facility in King County, Washington state, USA was done to evaluate the utility of symptom screening for identification of the virus. Of the 23 who were positive at the time of testing, 10 (43%) had symptoms, and 13 (57%) were asymptomatic.
Ten of 13 asymptomatics developed symptoms a week after the test.
The researchers concluded that symptom-based screening could fail to identify about half of the residents with Covid-19.
The China Health Commission started to publish daily data of asymptomatic cases from 1 April 2020. The asymptomatic accounted for 133 of 166 new infections i.e. 78% in the first day’s figures.
A South China Morning Post article, on 8 April 2020, reported that the China Health Commission stated that 601 of the 885 infections i.e. 68% reported between 31 March 31 and 7 April 2020 had no symptoms.
Researchers at the Centre for Evidence Based Medicine in Oxford, UK reviewed 21 reports from a search of LitCovid (a subset of the PubMed library system that tracks scientific information about the virus), medRxiv, Trip, Scholar and Google and reported on 6 April 2020 that
• “Between 5% and 80% of people testing positive for SARS-CoV-2 may be asymptomatic.
• Symptom-based screening will miss cases, perhaps a lot of them.
• Some asymptomatic cases will become symptomatic over the next week (sometimes known as “pre-symptomatics”).
• Children and young adults can be asymptomatic”
and “that there is not a single reliable study to determine the number of asymptomatics. It is likely we will only learn the true extent once population-based antibody testing is undertaken.”
Two recent publications in the New England Journal of Medicine (NEJM) reported on significant percentages of the asymptomatic and pre-symptomatic in New York, USA and Iceland.
The findings from universal screening for SARS-CoV-2 of 215 women admitted in labour to the New York–Presbyterian Allen Hospital and Columbia University Irving Medical Centre between 22 March and 4 April 2020 was published on 13 April 2020.
Swabs were obtained from 210 of 211 women who were asymptomatic. “29 of the 33 patients who were positive for SARS-CoV-2 at admission (87.9%) had no symptoms of Covid-19 at presentation.” Fever developed in 3 (10%) before postpartum discharge (median length of stay 2 days)”.
The findings of population screening in Iceland was published on 14 April 2020. “Symptoms were common both in participants who tested positive and in those who tested negative for SARS-CoV-2 in the overall population-screening group. Notably, 43% of the participants who tested positive reported having no symptoms, although symptoms almost certainly developed later in some of them.”
In short, there is increasing evidence that a certain percentage of those positive for SARS-CoV-2 are asymptomatic or pre-symptomatic. They are currently largely unassessed since testing is performed predominantly on those with symptoms or who are contacts of those found positive.
The Health Ministry stated on 13 April 2020 that the test positivity rate was 0.5% when all residents living in EMCO (Enhanced Movement Control Order) areas were tested. However, no other details were made available.
The Selangor Mentri Besar stated that tests on 649 samples at the Dusun, Sungai Ramal and Kajang state legislative assembly halls revealed that five were positive i.e. a test positivity rate of 0.77%. Four (80%) of the five positives were asymptomatic. However, no other details were made available.
In view of the paucity of data provided, it would not be scientific to assume that the positivity rates in the EMCO areas, wherever it might be, or the Selangor areas, were representative of the rest of the country.
However, 0.5% and 0.77% of the population of 32 million could translate into about 160,000 and 250,000 persons. Compare that with the numbers of positive cases reported daily.
Some people have tested positive, but given the limited supply and conduct of tests, that is probably a far cry from the actual number of people who have been infected.
Universal testing is not possible because of insufficient supplies, infrastructure, and human resources.
However, there is a less costly approach, i.e. testing a representative sample of the population.
Getting a viable representative sample from scratch would require much effort. What can be done, however, is to obtain a representative sample from the participants in the National Health and Morbidity Survey.
More accurate data will provide information about the actual numbers of infections. A more accurate multiplier will improve decision-making in the development of a suitable exit strategy from the Movement Control Order and to cushion the socio-economic impact of Covid-19.
If the multiplier is high, it would imply that a larger segment of the population is infected but are asymptomatic or having mild symptoms, indicating that herd immunity may not be far off.
In the final analysis, policies have to be based on evidence and customised to the realities on the ground. Some countries, for example, China, Luxembourg have commenced studies into asymptomatic cases of Covid-19. It is time for Malaysia to do likewise.
Dr Milton Lum is a past President of the Federation of Private Medical Practitioners Associations, Malaysia and the Malaysian Medical Association. This article is not intended to replace, dictate or define evaluation by a qualified doctor. The views expressed do not represent that of any organisation the writer is associated with.
- This is the personal opinion of the writer or publication and does not necessarily represent the views of CodeBlue.