Weather of Arabia - In June 2009, an 11-year-old boy from the United Kingdom returned to the United States, and within days, swollen salivary glands appeared while attending an activity in Sullivan County, New York. The boy had mumps, an infectious disease that spreads through droplets in the air, and the boy unknowingly transmitted the infection to the United States.
At the end of the activity in which students engage in daily group activities face-to-face around a small table, 22 children and three adults were infected. When these students returned home, the virus spread to Brooklyn and Rockland County, then Ocean County and Orange County, and a total of at least 3,502 people were infected over the course of a year.
Surprisingly, however, the boy who inadvertently spread the infection had received all the booster doses of the MMR vaccine. It is true that the boy was immune to the disease, as he showed relatively mild symptoms, but he was a carrier of the virus and a spreader of infection.
A study revealed that most vaccines do not completely prevent infection, although they prevent the appearance of symptoms. Therefore, people who have been vaccinated may carry and spread pathogens without knowing it. They may also cause a pandemic.
Vaccines provide two types of protection:
Sterile immunity: that is, the immune system is able to completely prevent infection, even if it is asymptomatic. But this kind of protection is rarely achieved by vaccines.
Unlike active immunity that relies on white blood cells such as B cells (B) and T (T) cells, and antibodies, sterile immunity depends entirely on neutralizing antibodies, which bind to the surface of pathogens, including bacteria or viruses, in the event of an invasion of the body and prevent them from entering the body. entering cells.
In the case of the emerging corona virus, the neutralizing antibodies that recognized the virus bind to the sticky surface protein on its surface that it uses to enter cells. To achieve aseptic immunity, vaccines stimulate the production of neutralizing antibodies to attack any viral particles that enter the body.
Until now, scientists have not evaluated the effectiveness of the available corona vaccines according to their ability to prevent transmission, but the main criterion in evaluating their effectiveness is the extent to which they can prevent the appearance of symptoms.
Studies have already proven that the antibodies produced by the body after a natural infection with Covid-19 disease do not prevent reinfection. A British study of health care workers concluded that 17% of people who had antibodies to the virus at the start of the study became infected a second time.
But there is early evidence that some vaccines may be able to reduce the transmission of infection in many ways, including reducing the number of viral particles in the body, as some assume that as long as vaccines prevent the appearance of symptoms of the disease, the number of viruses in the body will be less than usual, Hence, the chances of transmitting infection will be reduced, but this is only an assumption.
The effectiveness of vaccines in preventing transmission will affect many things, not only the extent to which the rules of social distancing continue to be applied, but also the achievement of herd immunity.
“If the vaccine does not prevent transmission 100%, the minimum number of people required to be vaccinated will rise to achieve herd immunity and eradicate the disease,” says Michael Head, Research Fellow in Global Health at the University of Southampton.
Herd immunity is defined as the indirect protection acquired by the population of a community from infectious diseases when a sufficient number of its members are immune to the disease. The minimum number of people required to be vaccinated to achieve herd immunity depends on many factors, including the rate of virus reproduction, meaning the number of new cases that a single infected case can transmit the infection to. This rate varies from one society to another, depending on the geographical area in which you live and the rules imposed to limit the spread of the virus, such as quarantine.
Therefore, scientists will not be able to set a certain minimum level to achieve herd immunity. But a study indicated that achieving herd immunity in a community will require vaccinating between 60% - 72% of the population, if the vaccine is 100% effective in preventing transmission, or between 75% - 90% of the population if the vaccine is 80% effective in preventing cross infection.
But some scientists do not expect to completely eradicate the virus. Rather, the goal now is to reduce the rate of transmission to a minimum. “We may see outbreaks of Covid-19 even after the vaccinations are distributed, but they will be in specific areas and will not spread globally,” Head says.
Some scholars believe that focusing on preventing transmission is a waste of time and effort. They justify this by saying that even if a person is able to spread the virus, once a large proportion of the population gets the vaccine, most people will be immune to the disease.
However, the issue of preventing transmission can be very important for people who cannot get the vaccine, such as pregnant women, young children or the sick. So in order to find a definitive answer, perhaps we should consider the story of the boy who spread mumps in the United States, and pretend we didn't get vaccinated.
Source: BBC Future
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