By Robert L. Moore, MD, MPH, MBA, Chief Medical Officer

“Don’t accept the world as it is. Dream of what the world could be – and then help make it happen.”

-Peter Tatchell

For the past year, for some reason I have enjoyed playing some Pandemic-related games with my sequestered family. The best is Pandemic Legacy: Season 1, a cooperative game where players work together to save the world from dangerous pathogens, including some that turn people into virtual zombies. It is a long game, taking many days to complete. I’m happy to say that my family saved the world twice, although many people died and some cities were destroyed in the process.

An older computer game (from 2008) called Pandemic 2 takes a different perspective, where you are the microbe and you are trying to kill as many people as possible. Not surprisingly, perhaps, the best way to do well in this game is to know how COVID-19 unfolded, with asymptomatic respiratory spread, followed by mutations that increase infectiousness and subvert the effectiveness of vaccines. This really helps hit home what is currently happening with the Delta variant.

Recent estimates put the R₀ more infectious Delta variant of COVID-19 at six to eight, compared to the R₀ of the original Wuhan strain, which was estimated at 2.7. (Reminder: the R₀ represents the average number of contacts infected by a single infected person.) This translates to 80-90% of the entire population (including children) would need to have good immunity to this strain to prevent a wave.

While the vaccination rate among California adults is 61.4%, only 51.8% of California’s entire population has been fully vaccinated against COVID-19. Of the 12% of the partially vaccinated, some may have had prior COVID infection before vaccination, in which case the single dose is probably about as effective as two doses in a person who never had COVID; the remaining individuals with a single dose have about 34% protection. Overall an additional 8% protection rate is reasonable. From seroprevalence studies, around 15% of the remaining 36% have been previously infected against COVID, but not vaccinated. Unfortunately, prior infection with non-Delta strains confers only about 30% protection against Delta, so the effective rate of protective immunity is only about 65% in our state, far from the now-needed 80-90%, hence the current exponential growth of infection against the Delta variant.

As of last week, the California Department of Public Health (CDPH) projected that the 2021 summer delta wave will be a little less severe than last summer’s COVID-19 wave. Based on the experience in the similarly-vaccinated United Kingdom, which is about 1 month ahead of us in the Delta wave, the hospitalization rate will be about 1/3 of what we experienced in prior waves. This means the risk of overwhelming the hospitals and ICU capacity of the state is low, so dramatic stay at home orders or restaurant closures are less likely to be needed or considered.

The upshot: this will likely be a quicker, steeper wave, with hospitalizations and deaths concentrated in the smaller, unvaccinated population. The rates of infection in the unvaccinated population will likely equal the overall infection rates we saw this past winter.

The vaccinated population is considering the recommendations of local public health officers to encourage wearing masks in indoor settings when among strangers, to slow down their lower-morbidity spread of the Delta variant. By itself, this will have a small impact on the overall epidemic curve. It may protect them and their families in the month ahead.

Vaccination is our best hope for fighting this particular virus, with its combination of high infectiousness, high level of asymptomatic spread, and relatively rapid development of mutations that help it evade our immunological and public health defenses—a lesson confirmed by two games written before COVID-19 struck.

You play a key role in this real-life game. Thanks for dedicating yourself to this vision of a better world.