a Covid-19, unlike the flu, the disease is completely new. Until this year no one in the world was to him no immunity. This means that even with the same level of mortality, this coronavirus threatens to kill more people. But for herd immunity need to be infected with at least 60% of people.
The epidemic of the coronavirus to last long. This is supported by a number of new studies from around the world to determine the exact number of cases.
Official statistics often underestimates the number of cases. But even in the new studies with more extensive tests, the infection rate remains within 10%. The exact threshold group (or “herd”) immunity was not yet clear, but some experts believe that it will be not below 60%.
The proportion of the population with antibodies to coronavirus
New York to 19.9% (may 2)
London is 17.6% (21 of may)
Madrid was 11.3% (may 13)
Wuhan 10% (20 APR)
Boston to 9.9% (may 15)
greater Stockholm is 7.3% (may 13)
Barcelona of 7.1% (may 13)
Studies show that even in the most affected cities of the world, the vast majority of people for the virus are still vulnerable.
Some countries — particularly Sweden and some time in the UK — abstained from mass quarantine in an attempt to produce population immunity. But even there, the infection rate ranges from 7% to 17%. In new York, where the largest recorded outbreak of coronavirus in the United States, as of early may, has infected about 20% of the population, according to the municipality based on the samples taken in shops and public places.
Similar studies conducted in China, where the coronavirus was born, but results are not yet published. However, as shown by a study in a hospital in Wuhan, the virus had about 10% of those who want to return to work.
Collectively the research suggests that in the near future, collective immunity, we are unlikely to achieve, according to official epidemiological school named Chan at Harvard University Michael Mina (Michael Mina).
The threshold of herd immunity to the new disease has not precisely defined, but many epidemiologists believe that he will come not earlier, than will be infected from 60 to 80% of the population will develop resistance or body resistance. A lower level of immunity among the population can slow the spread of the disease, but completely eliminate outbreaks, you will only achieve population immunity.
“to be Honest, secure way to reach him we would, at least, not in the shortest possible time, says Dr. Mina. Otherwise you will have to remove all restrictions and let the bug going around, but I think, society has decided that this option does not suit us”.
New research focuses on finding antibodies in the blood of people, this is produced byavaimia immune system proteins indicate a migrated infection. The advantage of this method is that it covers asymptomatic carriers who do not even know that you have been ill. Drawback — these tests are sometimes wrong. Several studies, including one from the acclaimed California already quarreled for error and selectivity.
Serological studies with representative sampling is conducted across America and around the world.
These studies are far from perfect, recognized biology Professor at the University of Washington Carl Bergstrom (Carl Bergström). But on set, he says, they give a more adequate idea of how widely had spread coronavirus — and what is the potential for further epidemics.
According to him, the threshold of population immunity varies from place to place and factors such as population density and social interaction. But on average, you will need a minimum of 60% immunity level. If the disease spreads easier than is commonly believed, this number may be even higher. On the contrary, with a wide variation in the risk of infection, this figure may be reduced.
All estimates of population immunity based on the fact that previous infection protects people against recurrence. There is strong evidence that the coronavirus produced by the immune system, — but how strong and for how long, is unclear.
Dr. Mina from Harvard suggests that group immunity is a kind of return fire, which slows down the spread of the disease.
He says that the patient, on entering the room to a healthy, can infect two or three people.
“on the other hand, if three of the four are already immune, infected on average by one person, or even less,” he explains. This person in turn will also infect less people. Thus, the probability of a major outbreak decreases.
But even when the collective immunity some people still get sick. “Your personal risk through contact with patients does not change, says Professor of epidemiology at the Johns Hopkins University Gypsyamber D'Souza (Gypsyamber D'Souza). — But decreases the risk of contact.”
Diseases like measles and smallpox, which once was sick, all children now in the United States came to naught, vaccines have helped to develop a sufficient population immunity to contain outbreaks.
Vaccines against coronavirus have not, so on the way to population immunity without new, more effective treatment will be more cases and more deaths.
Assuming that population immunity occurs at the level of 60%, it turns out that new York is still two-thirds of the way. And yet the mortality rate is 250 per 100,000 new Yorkers. Millions of citizens are still unable to be infected, and tens of thousands could die.
“Is there anyone who will undertake to advise others that what was new York?— says associate Professor of biostatistics at the University of Florida Natalie Dean (Natalie Dean). — Many are now talking about a “controlled infection" youth. But count on the control — presumption. To control the infection very difficult.”
The virus affects the population unevenly, communities with low incomes and ethnic minorities were hit harder. According to testing for antibodies, in some areas of the Bronx and Brooklyn, the incidence is twice higher than in the whole of new York, announced on Thursday, state Governor Andrew Cuomo (Andrew Cuomo). These areas are already close to the threshold of herd immunity, in which new outbreaks are becoming less likely. But since they are not isolated from the rest of the city, where the level of immunity is much lower, the residents remain at risk.
In other cities the proportion of people with antibodies according to serologic studies, is much lower. The quality of these studies varies somewhat, or the sampling was not completely random, either the tests are not accurate enough. However, a number of studies have shown that in most places the sick at least ten times more (and therefore deaths too) — and only then will be able to reach the point where new outbreaks can be deleted.
More serological studies are helping scientists to establish the mortality of the virus — how dangerous he is. So far, all mortality estimates are approximate. To accurately calculate them, it is important to know how many people in a particular place have died from the virus and how many were infected. Reported incidence rates based on the results of testing and do not always reflect the true extent of infection. The true scope of the outbreak identify serology.
In new York, where on may 2 according to the testing antibody were infected up to 20% of the population, the mortality was about 1%, has killed more than 18 thousand people.
For comparison, the death rate from influenza is estimated at 0,1-0,2%. The counting of seasonal flu cases are not as accurate as the serological tests. In addition, the number of infected is often underestimated, and the number of deaths on the contrary, overestimated.
But even if the mortality rate is exactly the same covid-19 as the disease is much more dangerous than the flu. The risk of sickness and death for the epidemic is much higher.
Each season flu risk nearly half of the population. Many already have some immunity, or they already had a similar strain of flu, or did the right vaccine, which coincided with the strain this year.
Fraction for population immunity is not enough, the flu comes every year. But people have ��preimushestvo partial immunity: in a normal year, at risk only part of adults, and the rate of distribution below. It turns out that the risk of death also decreases.
Covid-19, unlike the flu, the disease is completely new. Until this year no one in the world was to him no immunity. This means that even with the same level of mortality, covid-19 threatens to kill more people. One percent of a large number of cases is higher than the same percentage of a smaller number.
“Not all of the 328 million Americans are at risk with the beginning of flu season every fall, says associate Professor of epidemiology at the University of California, Irvine Andrew Noymer (Andrew Noymer). But as much at risk when there is a new infection.”
