Public health experts, when recommending the target number of cattle in a herd that must be protected from COVID-19 to clear the virus, provided a wide range of estimates, ranging from From 60% to 90% of the population. Massachusetts has set a goal of vaccinating approximately 75% of the state’s adults (4.1 million residents), but this accounts for only 60% of its entire population (including children). This is an ambitious goal, but no one is sure if it is sufficient to reach the threshold.
All the reasons for uncertainty? Regarding viruses and vaccines, we still have a lot of unknowns.
The immunization of cattle itself is quite real, and we have already seen its effect: According to data from the World Health Organization, after about 95% of the population was immunized, measles has basically disappeared in the United States. Epidemiologists even have a formula to calculate how many people must be vaccinated to gain immunity against any particular virus.
In an interview with The Globe, public health experts explained why this concept is so tricky. But they also emphasized that even if the virus cannot be completely eradicated, a large part of the population must still be immunized.
Dr. Anthony Fucci, director of the National Institute of Allergy and Infectious Diseases, suggested that 70% to 85% of the population should be vaccinated to make them fully resistant to COVID-19.
The huge gap is due to a year-long effort to fully understand the key characteristics of COVID-19: how contagious the disease is when people do not take preventive measures (such as wearing a mask or staying away from society).
Lipsch said that the whole world has never known this number clearly.
He said that in the early days of the pandemic, many scientists began to believe that with COVID-19, each infected person would infect 2.5 to 3 other people. If the number is 3, then according to the mathematical formula, this means that to achieve group immunity, about two-thirds of the people need to be vaccinated.
But scientists may underestimate the brutal efficiency of COVID-19 because early testing is so limited that many people who should have been positive may have been missed. Since then, it has been impossible to obtain more precise numbers because too many people have changed their behavior to avoid getting COVID-19.
Lipsitch said: “We are stuck in bad data.” “And, there is no really good way to overcome it, because, appropriately, we did not take the time to measure this number. We are taking the time to try to reduce it.”
The emergence of variants of the coronavirus also complicates this situation because it is generally believed that they are more infectious than the original form of the virus.
The higher the infection rate, the higher the herd immunity threshold. For example, if everyone can infect four people, three-quarters of them need to be immunized to reduce the virus. Five people will need 80%, and six people will need nearly 85%.
Dr. Virginia Pitzer, a professor of epidemiology at Yale University’s School of Public Health, said: “The threshold for getting immunized cattle is always incalculable.”
Water and fire
Further obfuscation of mathematics is another uncertainty: whether the vaccinated person can still spread the disease to others, thereby potentially harming those who have not yet been immunized.
Lipsitch said: “If you think of people as fire, and vaccines are like water trying to put out a fire, then we don’t know how wet everyone will be when using a single vaccine.” “Whether they are so wet that they never get wet. Catch fire, or if they get wet, it’s hard to catch fire, they don’t burn so much.”
Scientists are still exploring this question, and it may take some time to get a reliable answer. Some studies have shown that vaccines (which have been shown to greatly reduce illness, hospitalizations and deaths) can also significantly reduce transmission, but may not reduce it completely. Rochelle Walensky, director of the US Centers for Disease Control, said this week that the data so far is very promising.
Lipsitch suspects that vaccines can reduce disease transmission by up to 80%-he called it “quite good.” However, if the vaccinated people can still spread the disease to some extent, then it will further increase the threshold for eliminating the virus.
Lipsitch said: “It’s a pretty good number, but it’s pretty good and it does make things harder.” If the vaccine only eliminates about 80% of the spread of the virus, then the real herd immunity target may be “about 100%. .
Is the goal even realistic?
If eradication of COVID-19 requires immunization of the entire population, it may not be feasible at all.
Even if the threshold is low, many people may not schedule appointments. And there is no vaccine for people under 16 years of age.
Pitzer said: “I think it is almost impossible to reach the threshold of herd immunity, especially if the children are not vaccinated.” “If we want to immunize every person of age in the United States, We will not be able to achieve herd immunity.”
Another factor may help, but: Millions of people have been infected by COVID-19, which has brought people more immunity. Although many of them may also be vaccinated, some may eventually increase the number of people vaccinated.
Lipsitch said: “In the short term, this will be our friend.” “This will not last forever, because the more we reduce the chance of spreading, the less immune the birth will be, or the immunity of people will be reduced. Will weaken.” Therefore, in the long run, it does not solve the problem. But in the short term, this is very helpful. “
Lipsitch and Pitzer both emphasized that despite these obstacles, it is still very important to vaccinate the majority of the population.
Although the virus may not be completely eradicated, large amounts of vaccines will still greatly slow down its spread, enough to restore normal life. If the virus is still spreading, protecting those most vulnerable to the disease becomes especially important.
“If we can turn it into a disease that is harmful to humans, but for a small number of people, for the hospital system, the scale is small, then we will decide to be a society like the flu…. Try to reduce lives, But without destroying life, we have destroyed life so far and cannot control life.” Lipsic said. “We should not despair.”
You can contact Adam Vaccaro at firstname.lastname@example.org. Follow him on Twitter @adamtvaccaro.