What we know about asymptomatic cases of Covid-19 

1. What is an asymptomatic carrier?

An asymptomatic carrier is a person who is infected with SARS-CoV-2 (tests positive for it), which is the virus that causes Covid-19, but displays no clinical symptoms. There are two possibilities: 1. Some people never develop any symptoms throughout the course of the infection, and the body just clears the virus on its own. 2. Some people develop symptoms later  —  the pre-symptomatic phase is called the incubation period, which is usually between 2 and 14 days.

Here we are talking about the first category: those who never develop symptoms.

2. Can asymptomatic carriers of SARS-CoV-2 pass it on to other people?

Yes.

3. How many asymptomatic cases out there?

We don’t know. But here is a summary of some of the existing evidence.

• A study based on screening results of 565 Japanese citizens evacuated from Wuhan estimates the ratio at 30.8%.
• A study on the Diamond Prince cruise (not peer-reviewed) estimates the ratio at 17.9%.

(Note that when the screening is aggressive, the number of true asymptomatic carriers can be overestimated, because some people might test positive when they are in the incubation period. The share of asymptomatic cases on Diamond Prince was around 50% at some point, but some developed symptoms later.)

• In South Korea, where over 300k tests have been performed, over 20% didn’t show symptoms during hospitalization.
• According to SCMP, “classified Chinese governmental data” shows about 1/3 of all detected cases in China are asymptomatic.
• Chinese health authorities did not disclose the number of known asymptomatic cases to the public prior to 31 March. Since 31 March, the National Health Commission (NHC) began to disclose the daily active number, but not the cumulative number. However, one province  —  Liaoning Province reported the cumulative number on 3 April. According to Liaoning Health Commission, the province has 141 cumulative confirmed cases (excluding asymptomatic ones) and 56 cumulative asymptomatic cases as of 2 April. Although a very small sample, the ratio then is 28.4%.
• In Iceland, as of 18 March, 218 positive cases were identified and “about half of those who tested positive were non-symptomatic”. But we don’t know how many of these are pre-symptomatic.
• On 30 March, the director of the US CDC warned that “as many as 25 percent of people infected with the new coronavirus may not show symptoms”.

Taken together, most existing evidence seems to suggest between 20–30%.

4. How are asymptomatic cases discovered? 

In South Korea and Iceland, mass testing.

In China, the NHC says there are four main ways: 1. monitoring of close contacts; 2. investigation of clusters; 3. investigation of source of infection; 4. screening of population groups who have been to high-risk areas.

More recently, a few publicly reported cases were discovered through “get back to work” screenings (employers ask employees to test for coronavirus or offer free tests before they can get back to work). In Henan Province, three doctors tested positive without displaying symptoms in the screening organized by the hospital they work at in order to resume normal healthcare services. A resident in Wuhan tested positive without symptoms on 17 March after his employer required all employees who had infected family members to provide testing proofs. This resident’s father was diagnosed in January. He was not tested back then because of shortage of testing kits in January.

5. What is China’s policy on reporting asymptomatic cases?

The NHC confirmed that asymptomatic carriers can be infectious in its “Covid-19 Diagnosis and Treatment Plan” (5th Edition), published on 4 February.

On 5 February, the NHC instructed that asymptomatic cases should be included in confirmed cases.

However, since 7 February, with the publication of the “Covid-19 Prevention and Control Plan” (4th Edition), asymptomatic cases have been excluded from the count of confirmed cases. They become counted as “confirmed cases” if symptoms are developed. The numbers are still reported to healthy authorities, but not disclosed to the public.

Since 31 March, the NHC has been disclosing the daily active number, but not the cumulative number.

6. What is China’s policy on the management of asymptomatic cases?

According to the “Prevention and Control Plan”, once detected, asymptomatic cases should be reported to the CDC system within 2 hours. Local health authorities should complete the epidemiological investigation for each individual case within 24 hours. Asymptomatic carriers should be put in centralized quarantine and monitored for 14 days. In principle, the quarantine can end after 14 days and if the person tests negative twice. But the quarantine period should be extended if they still test positive. The close contacts of asymptomatic carriers should also be quarantined for 14 days.

The Wuhan resident mentioned earlier shared his own experience as an asymptomatic carrier. He was sent to Jinyintan Hospital – the specialist infectious diseases hospital that has treated the most severe cases during the outbreak – and hospitalized for some 7-8 days even without displaying any clinical symptoms. He was given antiviral medication. After discharged from hospital, he was quarantined in a hotel. His mother was also quarantined in a hotel for 14 days and tested several times (negative).

7. Lessons?

Wear a mask.