Herd Immunity: What Happened In California?

California is currently thouight to be the most-improved US state for COVID, having reversed the late 2020 surge soon after the new year, with no new third surge as happened in the US northeast and Canada. But since Los Angeles County accounts for a quarter of the state's population, much of this success is attributable to LA County's performance. Although Los Angeles and San Francisco Counties are both in the least restrictive "yellow" tier, San Francisco's population is less than a million, while LA County's is over ten million.

The chart above shows cumulative totals for LA County cases and deaths, as well as case mortality rate, since the start of the pandemic. While the numbers, being cumulative, don't go down, they do show how the spread of the disease has been mitigated. More important, the dark curve at the bottom of the graph, reflecting cumulative deaths, shows a much more gradual curve than the light pattern at the top, which is cases. The overall case positivity rate of those tested, accorsding to a May 2 county press release, was More than 6,513,000 individuals tested simnce the start of the pandemic, with 18% of people testing positive. However, the current test positivity rate is well under 1%.

But LA County's statistics raise interesting questions. The first is why the COVID curve reversed so quickly after January 1, as shown at right. Vaccines were distributed only starting im mid-December 2020, supplies were limited, and they were concentrated among health care workers and long term care patients at that time. In addition, people needed two shots spaced over four weeks to be "fully vaccinated", since the J&J vaccine wasn't yet available. But the overall trend reversed within the first immunity waiting period. It's hard to think vaccine was the only explanation.

This leads to the question of herd immunity. So far, the received wisdom on herd immunity is reflected in this Johns Hopkins piece:

As with any other infection, there are two ways to achieve herd immunity: A large proportion of the population either gets infected or gets a protective vaccine. What we know about coronavirus so far suggests that, if we were really to go back to a pre-pandemic lifestyle, we would need at least 70% of the population to be immune to keep the rate of infection down (“achieve herd immunity”) without restrictions on activities.

The stimate of 70% to "go back to a pre-pandemic lifestyle" is actually at the low end of what we've seen from the public health establishment, with other authorities offering 80% or 90%. But the evidence we see in LA County is at variance with the overall theory in the link. The LA County graph at the top of the post, plus the county press release, gives overall positivity -- in other words, immunity as a result of infection -- at well under 20% of the population. A link in yesterday's polst gives the "fully vaccinated" percentage of LA County's population as 40% -- but that's right now. 40% plus under 20% is less than 60%. But the curve headed downward well before we approached 60%.

Dr Ferrer, the county health director, doesn't really disagree with this conclusion.

Asked why L.A. appears to be outpacing neighboring counties in reducing case rates, Ferrer credited residents and businesses for adhering to health guidelines. But she also said the winter surge in infections is also a contributing factor to the current low case numbers.

“We had a devastating surge here with huge numbers of people that got infected, and many of those people are still carrying around some natural immunity,” she said.

She goes on to warn about the danger of easing off on protective measures, but her own health department has just endorsed the CDC's guidance that it's OK to go without a mask outside, and the "yellow" tier relaxes the guidelines primarily for sports events and outdoor concerts, the sort of things that used to be verboten unless George Floyd was involved.

But the example of Texas, which has never had controls as strict as California, shows that even with greater laxity, it's been possible to show a steady downward trend in infections -- and Texas weather was much colder than California this past winter. But even Dr Ferrer acknowledges that an infection rate well under 20% of the population had a favorable effect on LA County's performance, leaving vaccine aside.

It's hard not to conclude that even with a compliance rate for masking and social distance in LA County generally acknowledged to be over 90%, there was an irreducible level of infections. Court-imposed relaxation on church attendence restrictions had no adverse effect on the pandemic, since people could go back to church roughly when vaccines became available, and increased church attendance has been part of the current COVID environment. (Has anyone in Canada looked at this?)

On one hand, I think the data shows that some level of disease transmission is simply unavoidable no matter what restrictions are imposed. But at the same time, the transmission leads to immunity in any case. Health authorities at this point are severely underestimating both the effect of the vaccine and the benefit of immunity from infection.