Covid-19: Herd Immunity (Jun 16)

According to OpenTable, restaurant reservations in the US increased to 33% of normal, as of yesterday. It varies a lot between states: Rhode Island is close to 100%, while New York, Hawaii and DC are still under 10%. Those numbers are probably a decent indicator for general levels of reopening. The sharp dip yesterday was because it compares to Father’s Day 2019.

The Rt tracker website now shows 19 states with growth multipliers above 1. Arizona leads the pack, while most states in the Northeast are near the bottom. New York was #1 for new cases during most of April and May, but it fell to #8. The action has shifted south and west. Specific numbers vary with each update, but the trend has been consistent.

JUNE 19 UPDATE: the Rt tracker revised their calculation method today. They now show 26 states with growth rates greater than 1 (daily infections increasing). Several states are starting to look very scary.

Since economies are still ramping up, the best guess is that the growth rate will continue to increase. At some point, states with a growth rate much above 1 will run into trouble. Presumably they will then take action to reduce infections.

That means the Rt rate will probably hang out close to 1, pretty much indefinitely. For every infection that ends (via recovery or death), a new one will take its place (on average). Of course there will be large variations in different places.

In a previous post, I talked about containment. It can happen globally if every country reduces infections to zero, until the virus disappears. That’s what happened with SARS and MERS, the two previous coronavirus epidemics. Sadly, it seems very unlikely that containment for this one will ever happen in the US, nor globally. Covid-19 is just too well established. A few island nations have reached zero cases, but that’s still not great. They constantly will need to quarantine all visitors, or risk fresh outbreaks.

For a non-contained disease, the only other end game is herd immunity, which occurs when a critical mass of people are immune. If non-immune people are rare enough, then the disease gradually stops spreading. The amount of immunity needed depends on a lot of factors, but it’s usually between 60% and 90% of the population.

With herd immunity, the disease may eventually die out completely. It may stick around as a minor problem. Or it may return in surges, as each previous wave’s immunity fades away.

There isn’t anywhere with herd-level immunity to Covid-19 right now. New York City is probably the closest, with about 20% showing positive antibody tests. Elsewhere in the US, it’s rarely over 5%.

Doing some very quick math, if 3 months of infections made 5% immune, then it will take 45 months to hit 75%. The curve will be flat, but very wide.

Of course, vaccines are a great way to increase herd immunity very quickly, without all the sickness and death. In the US, roughly 60% of children and 45% of adults got a flu shot in 2018/9. For Covid-19, a recent survey showed 49% who plan to get a Covid-19 vaccination. 20% are unwilling, and 31% not sure.

That probably isn’t enough to attain herd immunity in one swell foop, but it’s close. After a vaccine is available and after half the anti-vaxxers get infected, the US will hit herd levels of protection.

Dennis Kolva
Programming Director

Author: Dennis Kolva

Programming Director for Turtle Creek Software. Design & planning of accounting and estimating software.