Are We Approaching ‘Herd Safety’ With COVID-19? – Medpage Today

Despite the emergence of the latest Omicron subvariants -- BA.2, BA.2.12.1, BA.4, and BA.5 -- Massachusetts observed no significant excess mortality this spring, researchers said.

From February to June 2022, when cases from these subvariants were highest, there were 0.1 excess deaths per 100,000 person-weeks in the state, corresponding to 134 excess deaths (95% CI -921 to 1,189), reported Jeremy Faust, MD, MS, of Brigham and Women's Hospital in Boston and editor-in-chief of MedPage Today, and colleagues.

And this was despite at least 226,857 new cases of COVID-19 during the 18-week stretch, according to their research letter in Lancet Infectious Diseases.

The level of excess mortality represents a 97.3% drop compared with the 8-week initial Omicron wave, during which there were 4.0 excess deaths per 100,000 person-weeks, or 2,239 excess deaths (95% CI 1,746-2,733), and a 92.7% drop compared with the combined 26-week Delta and Delta-to-Omicron transition periods, during which there were 1.5 excess deaths per 100,000 person-weeks, or 2,643 excess deaths (95% CI 1,192-4,094).

"We are finally in a phase now where highly immune populations can start to shoulder COVID-19 waves without the guarantee of excess mortality," Faust told MedPage Today. "Before, a COVID wave meant we knew we would have excess mortality."

However, illness from COVID-19 and associated hospitalizations continue to occur. "It's not all just about excess death," Faust noted.

Rather than the much-discussed "herd immunity," the morbidity levels being back to nearly normal shows "herd safety," he said.

"This spring, so many people walking around had a recent immune-generating event, vaccine, booster, or infection," with 80% of the Massachusetts population being fully vaccinated, he added. "So now we have something to show for that, but we don't know how long it will last."

There have been previous drops in mortality since the pandemic began. The first time there were fewer excess deaths in Massachusetts was during February to June 2021, when vaccines were being rolled out. During that time, "the mean age of newly infected people dropped precipitously and prevalence among people older than 60 years was low," Faust and co-authors wrote.

Conversely, the drop in mortality from late February to June 2022 did not correlate with infections in younger people. In fact, the mean age of newly infected people was higher, suggesting that "in our highly vaccinated state, current levels of immunity are considerable, leaving many, if not most, individuals at high risk with substantial protection against the most severe outcomes of SARS-CoV-2 infection," the authors noted.

For this analysis, Faust and colleagues used population data from 2014 to 2019, as well as weekly mortality data from January 2015 to February 2020 provided by the Massachusetts Registry of Vital Records and Statistics, which was 99% complete for all study weeks. They applied seasonal autoregressive integrated moving averages to project the weekly number of expected deaths for Massachusetts for Feb. 3, 2020 to June 26, 2022.

Faust explained that other states, including but not limited to Alaska, Arkansas, Arizona, California, Hawaii, New Hampshire, New Mexico, Oregon, Vermont, Washington, and West Virginia, are reporting higher rates of excess mortality during the same time period. Data are currently unpublished, as they are incomplete, "but the complete numbers won't be lower," he said.

Ingrid Hein is a staff writer for MedPage Today covering infectious disease. She has been a medical reporter for more than a decade. Follow

Disclosures

Faust reported no conflicts of interest. One co-author reported multiple relationships with government sources and pharmaceutical companies.

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Are We Approaching 'Herd Safety' With COVID-19? - Medpage Today