Has the delta variant ended our shot at COVID herd immunity in California and Sacramento?
Whatever happened to herd immunity?
Once described as a kind of golden ticket out of the coronavirus pandemic, herd immunity refers to a state in which enough people in a community develop protection against a virus via vaccination or previous infection to the point that the entire population is considered immune. Earlier in the pandemic, epidemiologists often cited 60% to 70% of the worldwide population as the necessary goalpost for establishing herd immunity.
But after a summer of relative optimism, the delta variant is driving up COVID-19 cases across the country. While the super-spreadable version of the virus is hitting unvaccinated populations hardest, so-called breakthrough cases – when vaccinated individuals test positive for COVID-19 – are contributing to concerns that we’re losing ground in the fight against the coronavirus.
As the outlook for the pandemic that has already killed more than 64,000 Californians seems to darken once more, The Sacramento Bee talked with public health experts about herd immunity to understand what it means and how the thinking around it has evolved.
First, let’s take a look at the current pandemic state-of-play in the Sacramento area:
Vaccination Rates in the Sacramento region, California
All data according to latest updates from county health departments and U.S. Census Bureau population numbers.
Sacramento County: 49.2%
Placer County: 51.4%
El Dorado County: 48.6%
Yolo County: 51% (as of 7/11)
Statewide: 54%, according to the Centers for Disease Control and Prevention
COVID cases
All data according to latest updates from county health departments.
Sacramento County: 120,417
Placer County: 26,004
El Dorado County: 11,566
Yolo County: 15,252
California: 4.13 million
Post-Vaccination Infections
All data according to latest updates from county and state health departments.
Sacramento County: 2,311
Placer County: 487
Statewide: 41,279 (as of 8/1)
Why 70% immunity is not enough for herd immunity
When public health experts and officials first started tossing around 60% to 70% as the percentage of immunized individuals that would mark herd immunity, they were working off the original strain of the coronavirus, Brad Pollock, a professor of epidemiology and chairman of the Department of Public Health Science at UC Davis, explained.
With the emergence of the delta variant, which is better at propagating than the original strain, that number is going to have to get up to 80% or 90%, Pollock said. And those immunity rates need to climb quickly in order to make sure other variants don’t emerge before transmission flatlines.
“The more people that are infected, the higher the likelihood is that you’ll get a random mutation of the virus,” Pollock said. “The thing we worry about is that the next couple of mutations that might occur could somehow ... seriously impact the effectiveness of vaccines that we have on the market right now.”
The available vaccines have still proved to be highly effective against the delta variant, despite some slight differences with the original strain, Pollock added. But that might not necessarily be the case if transmission doesn’t drop enough to prevent a new variant.
When will COVID end?
It’s important to remember that even when using a higher metric for defining herd immunity, crossing an 80% or 90% immunity threshold doesn’t mean COVID will simply disappear, Maureen Johnson-León, a data equity specialist with the University of Texas at Austin COVID-19 Modeling Consortium, said.
“Even after a high percentage of the population … becomes protected, there’s still going to be some transmission,” Johnson-León said. “So the more important thing is to really focus on the local dynamics, and the level of community transmission to understand what measures somebody or a community of people have to take to continue to protect themselves.”
The bottom line comes down to the same key points public health officials across the country are constantly repeating: people need to get vaccinated. Johnson-León emphasized the need for vaccine equity; according to her research, people with higher socioeconomic status tend to have greater access to the vaccines. Transmission rates will still be a problem if vaccines aren’t within reach for everyone.
Pollock also said masking requirements should come back, particularly to ward off asymptomatic spread of the virus. But the end of the pandemic still largely rests on getting shots to unvaccinated populations.
This story was originally published August 10, 2021 at 5:00 AM with the headline "Has the delta variant ended our shot at COVID herd immunity in California and Sacramento?."