When the delta variant of SARS-CoV-2 hit the United States in summer 2021, it led to a larger-than-expected surge in cases, hospitalizations and deaths.
Scientists from Penn State found this lack of preparedness may have been partially due to overestimating the number of US citizens who were immune or partially immune to the virus.
The research is published in JAMA Network Open and was conducted by Maciej Boni et al.
According to the researchers, it had been previously estimated that a combination of prior infections and high vaccination rates had led to between 80 and 85 percent of southern New Englanders having immunity against COVID-19 infection by the time the delta variant began to circulate in July 2021.
But in the new study, the team found that the percentage of those immune was actually closer to 67 percent, leaving almost one-third of Rhode Island, Connecticut and Massachusetts residents still fully susceptible to infection.
This was likely due to underestimating the number of people who had both been previously infected and vaccinated.
They found that prior to June 2021, about 27 percent of vaccines given in southern New England were given to people who had already been previously infected with SARS-CoV-2.
This overlap of infection and vaccination led to an inaccurate picture of the percentage of the population that was still susceptible to infection, therefore throwing off projections of what the delta variant surge would look like here in the US.
The researchers said the findings can help shape future vaccination strategies.
According to them, the most trusted and classic approach to predicting future surges is to consider the number of people who already have some degree of immunity, either through vaccination or previous infection.
This helps determine how many people are still susceptible to infection and serious disease.
But, the team said calculating the percentage of the population that has been previously infected with COVID-19 has been difficult throughout the pandemic.
It can be challenging to estimate not only the number of unreported symptomatic cases but the number of asymptomatic cases, as well.
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