Until recently, it was believed that the SARS-CoV-2 virus was distributed mainly by people, who already have symptoms. But empirical evidence puts this hypothesis into question - both the study of real cases and mathematical modeling say that the opposite may be true.
Despite all the precautions , the epidemic is expanding and gaining momentum. Many began to wonder - maybe asymptomatic carriers of the virus also spread the infection? This would explain the difficulty of containing it: when a person has no symptoms, his interlocutor is more tempted not to follow the safety rules in an epidemic, and medical staff are less able to identify and isolate the patient.
The Internet is flooded with a variety of information that confuse the number of infections from asymptomatic carriers (this is precisely this indicator) with the number of people whose disease in principle proceeds with minor symptoms or without them. It comes to the claims that 48 has 75% of patients do not show significant symptoms.
There is no epidemiological evidence for such statements. We do not know how many people actually have an asymptomatic illness. We can only judge this indicator by specific cases or build mathematical models that predict this number. But judging by the words of experts in an interview for CNN , the figure is most likely higher than we thought.
For example, in Massachusetts, according to patients, at least 77 of a known patient became infected from a person who at that time was not yet showing symptoms .
On Tuesday, the Director of the Institute of Medical Virology in Frankfurt, Professor Sandra Ciesek, tested the passengers of one plane flying from Israel to Germany, in total 19 person. Seven of them tested positive for coronavirus. Four did not have any symptoms, but the viral load calculated on the basis of the analysis of smears taken from patients without symptoms was higher than three with characteristic symptoms. (Viral load is a measure of the severity of a viral infection, calculated by estimating the amount of viral particles in a given volume of body fluid, in this case, pulmonary secretion. A higher load means that someone is more likely to spread the infection among other people). The results of the study have not yet been officially published, but posted online, which, however, is characteristic of the articles now available on the new coronavirus, since the process of reviewing the official publication of the review takes a lot of time.
In another not yet published, but laid out on MedRxiv study, mathematical models are presented to predict the number of people infected without symptoms. The January and February epidemiological data from the Chinese city of Tianjin and from Singapore were used, and they suggest to us that many were infected by people at the pre-symptomatic stage. On Saturday, it became known about a joint Belgian-Danish study, which showed that from 24 to 66)% of 87 people infected in Singapore caught the virus from people without symptoms. For Tianjin (135 cases), this is the path of infection characteristic of 62 - 77% infected.
Canadian, Danish and Singaporean researchers on Based on the same data, it was suggested that on average the infection was transmitted further for 2, 50 - 2, 87 the day before the onset of symptoms.
The American Centers for Disease Control and Prevention (CDC) still have an opinion, that the disease is mainly spread by people with manifested symptoms.
It’s still useful to know that people with moderate symptoms By the manifestation of the disease (there are such) they can definitely have a high viral load, which makes the COVID epidemic - different from the atypical outbreak pneumonia in 2002 - 2003 years, when the pathogen was distributed by patients with a pronounced clinical picture.