Overestimating population immunity contributed to COVID Delta variant surge in US
By PENN STATE UNIVERSITY
When the COVID-19 delta
variant hit the United States in the summer of 2021, it led to a
larger-than-expected surge in cases, hospitalizations, and deaths. New research
suggests this lack of preparedness may have been partially due to
overestimating the number of U.S. citizens who were immune or partially immune
to the virus.
According to the researchers, who focused their analysis on
southern New England, 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 SARS-CoV-2 (the virus that causes COVID-19) infection
by the time the delta variant began to circulate in July 2021.
But in their new study, the researchers 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.
Maciej Boni, associate professor of biology at Penn State, said this was likely due to underestimating the number of people who had both been previously infected and vaccinated.
“We 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,” Boni said. “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 – published on May 26,
2022, in JAMA Network Open – can help shape future
vaccination strategies.
“If we’re running a crisis-situation vaccination campaign in
the future, we should avoid allocating just enough vaccines to push the
population to herd immunity,” Boni said. “Instead, we should purchase more
vaccines than we think we need, roll out the campaign as quickly as possible,
and push the total number of vaccinees well past the herd immunity threshold.”
According to the researchers, the most trusted and classic
approach to predict 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, Boni 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.
“When estimating population immunity, it’s also vital to
take vaccination rates into account, including how many people have been both
infected and vaccinated,” Boni said. “You can’t simply add the number of people
who have been infected with the number of people who have been vaccinated or an
overestimation of immunity will occur.”
For this study, the researchers gathered thousands of data
points, structured into 11 separate data streams, from Massachusetts,
Connecticut, and Rhode Island, including information on confirmed cases,
hospitalizations, deaths, and weekly vaccination numbers.
Then, the researchers created a model to estimate the
overlap of previous infection and vaccination. The model assumed that people
who knew they had already been infected did not get a vaccine between January
and May 2021, when vaccines were in limited supply. But it also estimated the
number of people who got vaccinated because they were unaware they had already
had COVID-19.
“Combining these factors meant that 27 percent of people
that lined up for vaccinations in spring 2021 were already antibody-positive
and just didn’t know it,” Boni said. “Of course, many people who knew they were
infected also got vaccinations, per CDC and DOH recommendations, meaning this
is a minimum estimate of vaccines that went to people who already had COVID
antibodies in southern New England. It is likely that population immunity was
overestimated in many other states as well.”
Reference: “SARS-CoV-2 Attack Rate and Population Immunity
in Southern New England, March 2020 to May 2021” by Thu Nguyen-Anh Tran, MSc;
Nathan B. Wikle, PhD; Fuhan Yang, MSc; Haider Inam, BSc; Scott Leighow, BSc;
Bethany Gentilesco, MD; Philip Chan, MD, MS; Emmy Albert, BSc; Emily R. Strong,
MSc; Justin R. Pritchard, PhD; William P. Hanage, PhD; Ephraim M. Hanks, PhD;
Forrest W. Crawford, PhD and Maciej F. Boni, PhD, 26 May 2022, JAMA Network Open.
DOI: 10.1001/jamanetworkopen.2022.14171
The Bill and Melinda Gates Foundation, NIH/NIAID Center of Excellence in Influenza Research and Surveillance, National Institute of General Medical Sciences, National Science Foundation, Centers 460 for Disease Control and Prevention, and Pershing Square Foundation helped support this research.