Are long-term asymptomatic people highly infectious? Now we can find out

Perhaps you still remember that not long ago, I wrote an article that detailed what asymptomatic infections are (search and read: How many "asymptomatic infections" are there around you?), and the article also left a big unsolved case - those who have been infected with the virus for more than 28 days and still have no symptoms, that is, those who I call "long-term asymptomatic infections", how long can the virus last in their bodies? Are they still contagious?

This is a very important question. If this question is not clarified, people's panic cannot be completely eliminated. The most worrying result is that if long-term asymptomatic infected people are still contagious, then society will face a very difficult ethical dilemma, that is, should long-term asymptomatic infected people be isolated? How long should they be isolated?

Now, the answer to this question has finally come to light, and many people are relieved. The people of Wuhan have once again made contributions to China and the world, and we have also used solid data to counter the rumors and frame-ups of many conspiracy theorists. Today I will tell you in detail how this hard-won achievement was achieved.

How long will long-term asymptomatic carriers continue to carry the virus? Are they contagious or how contagious are they? There is only one solution to answer these questions, which is to conduct large-scale nucleic acid testing in a population that has experienced an outbreak of COVID-19. The scale of the test must be large enough to obtain convincing data. In addition, a comprehensive screening must be conducted after a considerable period of time has passed in this population in order to determine the situation of long-term asymptomatic carriers.

The city in the world that best meets these requirements is Wuhan. Wuhan has a permanent population of about 11 million. If we can conduct continuous nucleic acid testing on these 11 million people and form a trend chart of the proportion of long-term asymptomatic infections since the lifting of the city lockdown on April 8, then we can convincingly answer the questions that people are most concerned about.

However, it is not easy to conduct nucleic acid tests on tens of millions of people. Since the outbreak of the new crown epidemic, the testing institutions in Wuhan have been working at full capacity and going all out for testing. Before May 14, the average daily testing capacity was around 50,000 people. At this rate, it would take about 200 days to complete the testing of more than 10 million people, which is too slow.

On May 14, the Wuhan government announced a seemingly crazy decision: to complete nucleic acid testing of the remaining 10 million people within 10 days. This sounds like a joke, 10 million people, that is, an average of 1 million people will be tested every day. What does this mean? The United States, which has the most powerful testing capabilities in the world, only tested about 270,000 people per day in May. So when this news first came out, many overseas self-media people were sarcastic and ridiculed it, calling it a great leap forward and a lie. However, Wuhan is serious this time. Starting from the 15th, the number of tests suddenly increased dramatically:

Not surprisingly, many people do not believe the authenticity of the data. In fact, I can understand the logic of those skeptics. The only problem is that they forget to search for scientific papers before questioning. In fact, it is entirely possible to increase the detection capacity. The key lies in changing the detection method. If we still test samples one by one as before, the detection capacity will certainly not increase 10 times instantly.

If we change our thinking and mix samples from many people together, if the test result is negative, then it means that all these samples are negative. If it is positive, then we can test this group of samples one by one to determine which sample is positive. In this way, wouldn't it be possible to greatly improve the detection efficiency?

This method sounds simple, so why wasn't it used before? The problem is that we don't know how many people should be divided into a group. If there are too many samples in a group, each group will most likely have to be retested, which will extend the testing time; and if there are too few samples in a group, the time for the first test plus the time for rechecking the samples one by one in the second test may not necessarily be shortened.

In fact, two of the world's four top medical journals published papers on this topic in early and late April, respectively.

On April 6, the Journal of the American Medical Association published a research paper titled "A sample-based strategy for detecting the spread of the new coronavirus in the population." Benjamin Pinsky, medical director of the Stanford University Clinical Virology Laboratory, and two other colleagues co-authored the paper. They used patient samples to test for the new coronavirus pneumonia virus to track the early spread of the virus in the San Francisco Bay Area. The researchers tested throat or lung airway samples from 2,888 patients. It had been confirmed in advance that the respiratory symptoms of these people tested negative for common respiratory viruses.

They divided the samples into groups of 9-10 and then tested the mixed samples for the presence of the new coronavirus. Of the 292 mixed samples, only two tested positive. Further analysis showed that one person in each positive group was infected with the new coronavirus. The laboratory officially began using this test method in early March. As of the publication of the paper, more than 12,000 samples have been collected from people with symptoms of new coronavirus pneumonia or known close contacts. Of these, about 9% tested positive.

Benjamin Pinsky

On April 28, The Lancet published a paper by Professor Sigrun Smola, director of the Institute of Virology at Saarland University in Germany [2]. This time, the researchers combined samples from up to 30 individuals in a test tube. Even if only one of the 30 people was infected with the new coronavirus, the researchers could still get a positive result through the mixed test. The researchers believe that only if a mixed pool has a positive result, a second phase of testing of individual samples is needed to identify the infected person. If the infection rate is low and many sample pools are negative, a large number of test tools will be saved and the testing capacity will be greatly improved.

Sigrun Smola

In this context, our expert team referred to foreign research results and combined with the actual situation, and suggested that the Wuhan government adopt a mixed sample testing mode with 5 to 10 people in a group. In this way, the detection speed can be increased by 5 to 10 times in theory. So, starting from May 15, the vigorous Wuhan battle began. The streets of Wuhan were full of long queues of people wearing masks and quietly waiting for sampling. This is a group of people who have experienced storms. What I see is the firm and self-disciplined look of the people. They have the unique temperament of Wuhan people after the disaster.

The Wuhan Health Commission publishes test results every day, and the situation of asymptomatic infections is becoming clearer and clearer.

The conclusion of big data statistics is: in early April, there were about 208 asymptomatic infections per 100,000 people on average; in early May, the number dropped to about 42; after another 10 days, the number dropped to about 8; as of now, there are less than 3 long-term asymptomatic infections per 100,000 people on average. In 50 days, the number of long-term asymptomatic infections has decreased by 99%.

In other words, as of May 24, 2020, there are less than 115 long-term asymptomatic infected people moving freely in Wuhan, and this number is decreasing every day. In the past 50 days, there have been almost no new local confirmed cases in Wuhan, which has well demonstrated that the infectiousness of long-term asymptomatic infected people is now almost zero and there is nothing to be afraid of. Everyone can put down their worries.

Image source: Phoenix News Client

As of the time of this article, the Wuhan battle has come to an end, and the trumpet of victory has sounded. According to public information, the battle cost about 2 billion yuan in total. This is indeed a large sum of money, but no one would think that the money was not worth spending. Because of it, countless Wuhan people posted photos on their WeChat Moments, holding up their mobile phones, with green QR codes on the screen and bright smiling faces on the side of the screen.

Image source: Yangtze River Daily Weibo

With this green QR code, they can travel around the country with peace of mind, without worrying about causing psychological pressure on others. In the past few months, I have seen too many sad stories, and I have also seen the helpless and angry expressions of many Wuhan people. After the battle is over, all this will become history, and Wuhan people can have a good "early morning" again.

References:

[1]https://jamanetwork.com/journals/jama/fullarticle/2764364

[2]https://www.thelancet.com/pdfs/journals/laninf/PIIS1473-3099(20)30362-5.pdf