Study: population immunity is overestimated, leading to a surge of covid delta variant infections in the United States
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According to the researchers, their analysis focused on Southern New England. Previous researchers estimated that when the delta variant began to be prevalent in July 2021, the combination of previous infection and high vaccination rates resulted in 80-85% of people in southern New England being immune to sars-cov-2 (the virus that causes covid-19).
But in their new study, the researchers found that the proportion of those immunized was actually close to 67%, and nearly one-third of residents in Rhode Island, Connecticut and Massachusetts were still completely vulnerable to infection.
Maciej Boni, an associate professor of biology at Pennsylvania State University, said that this was probably due to an underestimation of the number of people who had been infected and vaccinated before.
“We found that by June 2021, about 27% of the vaccinations in southern New England were for people who had previously been infected with sars-cov-2,” Boni said. “This overlap of infection and vaccination has led to an inaccurate description of the proportion of the population still vulnerable to infection, thus deviating from the prediction of a surge in delta variants here in the United States.”
The researchers say these findings could help shape future vaccination strategies. The study was published in the online edition of the Journal of the American Medical Association on May 26.
Boni said, “if we are going to carry out vaccination activities in crisis situations in the future, we should avoid only distributing enough vaccines to promote the population to reach group immunity. Instead, we should buy more vaccines than we think we need, launch the activities as soon as possible, and make the total number of vaccinators far exceed the threshold of group immunity.”
According to the researchers, the most reliable classical method to predict the future outbreak surge is to consider the number of people who already have some degree of immunity, whether through vaccination or previous infection. This helps to determine how many people are still vulnerable to infections and serious diseases.
However, Boni said that it has been difficult to calculate the proportion of the population previously infected with covid-19 throughout the pandemic. Estimating not only the number of unreported asymptomatic cases but also the number of asymptomatic cases can be a challenge.
Boni said: “when estimating the population’s immunity, it is also important to consider the vaccination rate, including how many people are infected and vaccinated. You can’t simply add up the number of infected people and the number of vaccinated people, otherwise the immunity will be overestimated.”
In this study, researchers collected thousands of data points from Massachusetts, Connecticut and Rhode Island, which were structurally divided into 11 independent data streams, including information on confirmed cases, hospitalizations, deaths and weekly vaccination.
The researchers then created a model to estimate the overlap between previous infections and vaccinations. The model assumes that people who know they have been infected will not be vaccinated between January and may 2021, when the vaccine supply is limited. But it also estimated the number of people who had been vaccinated because they did not know they had been infected with covid-19.
“Combined with these factors, it means that 27% of the people queued for vaccination in the spring of 2021 are antibody positive, just don’t know,” Boni said. “Of course, according to the recommendations of the CDC and the Ministry of health, many people who know they are infected have also been vaccinated, which means that this is the lowest estimate of vaccinating people in southern New England who already have covid antibodies. It is likely that in many other states, the population’s immunity has also been overestimated.”