(Yicai Global) March 30 – The Covid-19 epidemic has been brought under control in China and the country has resumed work and production. But occasional cases, especially those of people who are infected but have no symptoms, suggest that China should not relax its epidemic prevention and control measures.

The infection rate for close contacts of asymptomatic infection was 4.11 percent while the infection rate of close contacts of confirmed cases was 6.3 percent, a team led by Chen Yi from the Institute for Communicable Disease Control and Prevention of the Ningbo Center for Disease Control and Prevention said in a paper in the pre-published online version of the Chinese Journal of Epidemiology on March 28.

Investigations into asymptomatic infections are ongoing, academician Gao Fu, director of the Chinese Center for Disease Control and Prevention, told Yicai Global. Many experts also said that asymptomatic infections require attention, but people need not be overly nervous.

Jiang Rongmeng, chief physician of the infection center of Beijing Ditan Hospital, believes that there are infected asymptomatic people with a long incubation period but their numbers are very small, and the chances of further transmission is also very low. And if they show any symptoms, they can be found through the existing strict prevention and control screening system, which, as a result, will not cause a large-scale outbreak. There is no need for the public to be tested for the virus if they show no symptoms and have no history of close contact with infected people unless they are involved in the investigation.

In fact, not just Covid-19 but tuberculosis also has cases of asymptomatic infections, according to Li Liang, deputy director of the National Center for Tuberculosis Control and Prevention of the Chinese CDC. It's better to wear masks for a longer time as a prevention and control measure, he said, adding that we need to be cautious about lifting the requirement to wear masks in public.

Yicai Global raised eight questions concerning asymptomatic infections with Chinese infectious diseases experts for our readers' reference.

Question 1: What is asymptomatic infection? How do infections without symptoms develop?

Jiang Rongmeng: Asymptomatic infection refers to those people who show no clinical symptoms, no respiratory tract and other specimens of novel coronavirus etiology (usually referred to as nucleic acid detection) or are positive for serum-specific IgM antibodies, according to the definition in the sixth edition of the Covid-19 Prevention and Control Plan.

If a person is infected with the novel coronavirus, he or she may face two outcomes after a 14-day incubation period – symptoms may appear or not. Whether or not the symptoms appear, they may test positive in nucleic acid tests later in the incubation period, however. But, a positive test does not necessarily mean that it is contagious.

Usually, seven to 10 days after the onset of the disease is the most contagious period, and patients with a cough are more likely to emit the virus. They are more infectious. Asymptomatic patients are less likely to spread the virus and are less contagious. As protective antibodies are produced, the virus is eliminated and is not contagious.

China's diagnosis and treatment plan is for symptomatic infected people. Asymptomatic infected people have no symptoms and show no manifestations of pneumonia. They just test positive in nucleic acid or antibody test and cannot be considered patients. They are not clinical cases, so are not included in the plan.

Li Liang: Actually, it's not just Covid-19. Tuberculosis also has asymptomatic infected people.

But this concept needs to be clarified. First, the infected. Infected people mean that viruses and bacteria reach the body and survive. The infected person develops further to become a patient. Therefore, the infected have no symptoms without the onset of the disease. The concept is not the same. The definition of asymptomatic infection is a bit vague.

Generally speaking, there are three endings after the pathogen enters the human body. First, the human body is very resistant, kills the pathogen, and has no symptoms. The second is that the person turns into a patient after the pathogen, which is relatively toxic, multiplies in the body and creates a lesion. The third is the dynamic balance between human resistance and pathogens. Pathogens can exist in the human body for a long time. At this time, pathogens enter a dormant state.

Therefore, it takes time for 'infected' people to turn into patients after the pathogen enters the human body. For example, it takes at least eight months, or even 10 years or decades, for TB to transform an infected person into a patient.

In theory, asymptomatic patients are contagious. A throat swab with a positive nucleic acid test result means that the virus is already in the body, and the person has become a patient. A positive nucleic acid test means a confirmed case that must enter the infectious disease monitoring system.

Question 2: How do you find asymptomatic people? Do we need mass screening?

Jiang: Asymptomatic infections are almost impossible to detect since people usually do not go to hospital without clinical symptoms. They are mainly found through close contact screening, cluster outbreak investigation, and infection source tracking.

The proportion of asymptomatic infections (including recessive infections) of Covid-19 epidemic needs to be determined by community surveillance and seroepidemiological investigations and cannot solely rely on model predictions.

Question 3: How should we report and manage asymptomatic people, according to prevention and control requirements?

Jiang: The sixth edition of the Covid-19 Prevention and Control Plan requires that medical and health institutions at all levels report suspected cases, confirmed cases, and asymptomatic infections within two hours. Asymptomatic infections should be quarantined for 14 days. In principle, those who test negative for two consecutive samples of nucleic acid test (sampling interval of at least 24 hours) can be released.

The CDC should investigate and immediately verify after receiving the report. It should complete the epidemiological investigation of asymptomatic infection within 24 hours. Close contacts of asymptomatic infections should be put under 14 days of medical observation.

Asymptomatic patients with clinical symptoms should be promptly revised as confirmed cases. Not everyone needs to be tested.

Question 4: Is it necessary to conduct research on asymptomatic infections? How should people who are positive in nucleic acid and antibody tests be treated?

Jiang: It is necessary. Through community monitoring and seroepidemiological investigations, we can know the proportion of asymptomatic infections in key populations; how many are nucleic acid-positive; how many are only antibody-positive; how many nucleic acid-positive people develop into patients; and how many close contacts of asymptomatic infections show the onset of the disease.

People with positive nucleic acid test and no symptoms do not need excessive attention or special treatment. They just need to cooperate with medical staff in isolation and observation. If the infected are purely antibody positive, it means that they have generated immunity and need not worry about infection or getting sick.

Question 5: How do you prevent and control asymptomatic infection? Does everyone need to undergo nucleic acid testing?

Jiang: Covid-19 is a new infectious disease. No person is immune to it except for those who have recovered from the disease. In the absence of a vaccine, we can avoid infection by strengthening self-protection and cutting off the route of transmission.

Asymptomatic Covid-19 carriers are not easy to identify because they show no symptoms and thus do not go to hospital, but they can only infect others in close contact without any protection. Maintaining a safe social distance (1 to 2 meters) can prevent infection.

People with no symptoms and no history of close contact with Covid-19 patients do not need to get tested unless they are involved in the investigation.

For an infectious disease to become an epidemic, first of all, there must be a source of infection. Second, there must be a route of transmission. If a person is not exposed to pathogens, he/she is unlikely to be infected.

Here are three suggestions for the public:

1. Do indoor ventilation frequently: Ventilation can dilute pathogens.

2. Wear masks: Wearing masks can prevent pathogens from entering your body.

3. Wash hands frequently: Washing hands will wash out the virus. The virus cannot enter your body as long as your skin is not broken.

Covid-19 can spread through the mucous membranes of the eyes, nose, and mouth. Thus, besides washing hands before eating or after toilet, please also wash hands before going to the toilet. This is very important. It is very dangerous to rub your eyes, pick your nose, or touch your face without washing your hands. Not just Covid-19 but influenza is also spread this way.

The public also must be impressed upon to improve their health literacy, especially infectious disease literacy. It is recommended that if you have fever or cold, you must, for example, cover your mouth with the crook of your arm or tissue when coughing or sneezing. Never use your hands. Moreover, you must wear a mask when you go to hospital to prevent transmitting pathogens in your body, if any, to other people.

Li: I think it is very important to wear a mask and avoid going outdoors to prevent the spread of the disease. Covid-19 is transmitted mainly through the respiratory tract and also via contact. People with no symptoms but positive for throat swab nucleic acid testing are likely to spread the virus through the digestive tract and other methods, so asymptomatic patients cannot be ignored or discounted.

Question 6: How to treat asymptomatic patients?

Li: Asymptomatic patients need intervention treatment if they are very likely to have symptoms. We don't have to be particularly nervous about asymptomatic patients. Whether or not they have symptoms depends on many factors. I think asymptomatic patients must be put under strict observation.

Question 7: Are nucleic acid-positive asymptomatic patients contagious?

Li: Any person testing positive is contagious. I think all patients, with symptoms or not, are contagious, the difference is in the transmissibility of the virus.

So it is very important for patients to wear masks and avoid going outdoors to reduce external transmission. It is equally important for ordinary people to wear masks, because you don't know whether people around you are patients or asymptomatic patients or not. So wearing a mask is the safest way to protect yourself in this case, which is low-cost and effective. Therefore, I suggest we should wear masks as long as we can.

Moreover, there are still many imported cases from abroad. We need to be prudent when considering allowing the public to get rid of masks.

Question 8: Will asymptomatic Covid-19 carriers cause a second outbreak?

Jiang: Close contacts of confirmed cases and related people have been put in quarantine according to the virus prevention and control actions nationwide, which is the best way to deal with asymptomatic patients. We may have missed out on asymptomatic patients with a very long incubation period but the number is very small, and the chance of further transmission is also very small. And they can be identified by the strict screening system for virus prevention and control if they show symptoms, and the timely isolation or quarantine of their close contacts will ensure large outbreaks won't happen.

Li: Asymptomatic patients are contagious but much less so. Therefore, I think we must pay attention to but do not need to be overly worried about such patients. Patients with symptoms and asymptomatic patients are different in how they spread the virus.

Editors: Liao Shumin, Chen Juan, Peter Thomas