(Bloomberg Opinion) -- As the summer began, a wide swath of states from the Northwest to the Southeast confronted the arrival of Covid-19’s Delta wave with, by my rough estimate, 35% or more of their residents fully vulnerable to the disease — that is, neither previously infected with nor vaccinated against it.
Sure enough, the darker-blue states on the map tended to have a tougher time of it this summer than the lighter ones, starting with Missouri, then spreading to the south and east and more recently the Mountain West and Appalachia. There were lots of other factors at work besides the vulnerabilities depicted on the map — state and local public health policies, behavior patterns, weather, bad luck. But the vulnerability almost certainly played a role. When I ran a regression analysis on the vulnerability percentages and the Covid-19 deaths per 100,000 residents since June 20 for all the states, the variation in vulnerability explained 16% of the variation in deaths, with only a 3.5% probability that the relationship was pure chance.
To calculate my unprotected/uninfected percentages, I started with the historical vaccination data from the Centers for Disease Control and Prevention and the cumulative infection estimates from covidestim.org, an invaluable creation of researchers at the Harvard and Yale schools of public health that “combines evidence on Covid-19 transmission, natural history and diagnosis with reported cases and deaths” to come up with its numbers. I assumed that the unvaccinated were just as likely to have been infected as the vaccinated, which surely isn’t the case (both because of the protective effects of vaccines and likely differences in behavior between the two groups) and probably skews my vulnerability estimates upward, but seemed simpler and more transparent than the alternatives.
When a higher percentage of the population has developed immunity to a disease, it spreads more slowly, with the herd immunity threshold the immunity percentage after which incidence of the disease inevitably declines. The formula for calculating the threshold is 1 - 1/R0, with R0 being the disease’s basic reproduction number, or how many people each person who catches it is likely to infect assuming no immunity and no behavior changes or government interventions to slow the spread. Last year Covid-19’s R0 was estimated at around 3, which worked out to a herd immunity threshold of 67%. The Delta variant’s R0 is closer to 6, which results in a threshold of 83%.
By my reckoning, more than 83% of the residents in five states (New Mexico, Arizona, Rhode Island, New Jersey and Florida) have now been fully vaccinated against or infected with Covid-19. That doesn’t mean those states have reached herd immunity: The many breakthrough infections over the past few months have demonstrated that, as expected, the vaccines don’t confer complete immunity against infection, and while the latest evidence points to infection-derived immunity being stronger (as well as a lot more dangerous to acquire), it, too, appears to fade over time. Still, with so few of their residents completely vulnerable to the disease, those states should find it much easier to slow its spread.
Nationwide, 54.9% of Americans have now been fully vaccinated and an estimated 51.6% infected, giving a vulnerable percentage of just 21.8%. It may still be that colder weather will bring yet another wave of Covid cases, but most of the country is starting to run out of people with no immune protection whatsoever. The Covid Scenario Modeling Hub, which combines the results of 11 different forecasting models, is currently not anticipating a big resurgence this fall and winter.
Some states still look pretty vulnerable, though. Among them are some of the usual low-vax suspects, but also states with pretty good vaccination rates plus better-than-average success in fending off Covid-19 so far.
Hawaii has the highest vulnerable percentage, at 34.5%. It has a modestly better-than-the-national-average full-vaccination rate, at 57.1%, but fewer than 20% of its residents have been infected, according to covidestim.org. As a result, it faces a less extreme version of what Australia and New Zealand, both considered Covid success stories, are now going through. The state’s governor last month urged tourists to stay away and warned that those who come “will not have the typical kind of holiday that they expect to get when they visit Hawaii.”
It’s certainly not the worst problem in the world to have — among U.S. states, Hawaii has had the second-fewest Covid deaths relative to population, trailing only Vermont. But it’s still a problem.
This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.
Justin Fox is a Bloomberg Opinion columnist covering business. He was the editorial director of Harvard Business Review and wrote for Time, Fortune and American Banker. He is the author of “The Myth of the Rational Market.”
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