The new study suggests that picked up the coronavirus, the person becomes contagious before he had symptoms, says Meduza. Therefore, the risk of developing COVID-19 is great for those who are in contact with perfectly healthy looking people.
We used to think that infectious diseases can become infected if you sneezed or even breathed obviously a sick man, or if you ate with him from the same plate. It is now clear that coronavirus infection is transmitted among people who are indistinguishable from healthy in appearance and well-being. Today the coronavirus was diagnosed in more than 2 million people worldwide. And almost half of them (44%) were infected from those who outwardly didn’t look sick, then there is reason to fear contact with such a man was not.
That is a sick person and he still does not know that is sick – he has no symptoms or they are so minor that he writes off their fatigue from work or a cold. Therefore the man or woman not go to the doctor, but continues calmly to communicate with relatives, to go shopping, walk their dog or to work as a courier… But all this time the person already infect others.
After studying the disease in 94 patients with confirmed coronavirus infection (critically severe patients was not among them) experts have calculated: the average, people begin to be contagious 2-3 days before the appearance of obvious symptoms. The peak’s ability to infect others drops out for a period of about 14-16 hours after the first symptoms. Then the ability to infected by the virus falls during the week, until complete disappearance. In other studies, scientists reported that the virus RNA can be detected in the blood up to 20 days or even longer (in some cases up to 37 days), however, its infectivity, apparently, do decreases because it becomes impossible to cultivate under laboratory conditions.To know the proportion of asymptomatic — or, more correctly, predictive of transmission of the virus, scientists don’t accidentally check any potentially infected people in the population, and statistically analyze the chains of transmission. Here, epidemiologists need to assess two key properties. First, the incubation period of the disease — that is, the time during which a person already infected, but the infection does not manifest itself. Second, the serial interval — the so-called mean time between the onset of symptoms in a chain of transmission: for example, if Alice had infected Bob, and it is known that cough Bob began three days later the cough Alice, three days — this is the serial interval.
Both parameters can be assessed independently from each other, and they can help to estimate the proportion of asymptomatic infections: if the serial interval in the population, on average, less than the incubation period, so a certain amount of infection occurs before symptoms begin to appear. And to calculate this proportion, you need to compare the distribution of these two values. This, however, is not enough: you need to evaluate how the obtained values are consistent with the amount of virus that is present in saliva and other secretions, and how these values change with time.
During the outbreak of another epidemic coronavirus, severe acute respiratory syndrome SARS/SARS in 2003, it was found that the peak ability to the infection, the patient was period 7-10 days after the onset of initial symptoms. Therefore, measures of quarantine that is most convenient to apply to obviously ill people, worked in this case very effectively. The flu, for example, often become severely contagious before there are major symptoms or at the beginning of their manifestation.
Published in Nature Medicine paper, the researchers analyzed the course of disease in 94 patients with confirmed coronavirus infection (none of them have been critically severe patients). Within 32 days after symptoms they took swabs from his throat. It was found that the peak concentration of the virus in the mucous membranes starts around the time of the appearance of the first symptoms of the disease and then declines and virtually disappears by around 21 days. It is not affected neither sex nor age, nor the severity of the human disease.
Attempts to estimate the real number of people infected from the asymptomatic carriers have done in Singapore and Tianjin. If in a paper published in Nature Medicine, the number of people infected from these media is estimated at about 44%, while in other studies this assessment was 48% and 62%. Scientists suggest that this may be due to the introduction of strict quarantine measures, which prevented the infection in Singapore and China in the later stages of disease each infected.
Data from the work published in Nature Medicine, cannot be considered absolutely complete. For example, the moment of occurrence of initial symptoms was retrospectively observed: this was told by the patients themselves after they were diagnosed. In addition, the researchers note that the more reliable information about the specific case of transmission, the longer was the serial interval, and this suggests that perhaps, on average, these intervals are somewhat understated. Finally, patients received treatment that could affect the concentration of the virus in the mucous membranes, and, accordingly, the rate of decline could also be understated relative to the non-drug variant of development of events.
However, scientists have shown that the average infected person is able, apparently, to infect others for another two or three days before symptoms appear, and that the concentration of virus in the blood is maximum on the day of occurrence of the primary symptoms. It is certainly worth to take into consideration: is it justifiable to consider yourself, your neighbor or the courier came perfectly healthy, despite the fact that the potential source of infection also does not suspect anything and feels good.
As previously wrote ForumDaily:
‘Learning to breathe again’: what happens with patients COVID-19 after resuscitation.
The ranking of economies on the effectiveness of anti-Covid-19: USA — among the outsiders.
Fox News: ‘patient zero’ with COVID-19 contracted in the laboratory of Virology of Wuhan.