COVID, immunity and vaccines (no politics, science question)

[tspear]

I have seen previous estimates that likely between 60 and 80% of the population needs to have "immunity" to break the epidemic. Most have stated there is not enough evidence to pin it down further yet.

With about 95% effectiveness for the first two mRNA vaccines you need between roughly 63 and 84% of the population to break the pandemic.

So two questions, First one; with the new strain out of Great Britain, which is some reports is almost twice as contagious. What does that do to the herd immunity numbers?
Two, vaccines like the Astra Zenica which are only 62% effective, could they ever "break" the pandemic?

Tim

 

[PeterNSteinmetz]

So two questions, First one; with the new strain out of Great Britain, which is some reports is almost twice as contagious. What does that do to the herd immunity numbers?
Two, vaccines like the Astra Zenica which are only 62% effective, could they ever "break" the pandemic?

In terms of the immunity numbers, according to the SIR (Susceptible-Infected-Recovered) model of disease spread, the herd immunity threshold is given by (R0-1)/R0, where R0 is the base theoretical reproduction rate of the illness. See https://academic.oup.com/cid/article/52/7/911/299077.

It is thought that R0 for Covid-19 may be on the order of 4, so that would correspond to 0.75. A higher R0 of 8 would correspond to 7/8 or 0.875. A substantial increase, but not as bad as measles with an R0 of around 16.

There are more sophisticated models, but a simple way to think about the effect of a vaccine is to reduce the population of susceptible (S) individuals and they can directly contribute to the 0.75 or 0.85 of (R) that are needed to achieve herd immunity. So a less effective vaccine just takes fewer people into the 'recovered' category in this model and would thus require more to be infected and achieve natural immunity.

 

[PeterNSteinmetz]

One thing that I never truly understood is the “risk factors”. If they have been treated and are now not a
“risk”, are the still a risk factor? For example, hypertension, diabetes, high cholesterol are all amenable to treatment with drugs and/or diet to bring things into a normal range.

Cheers

Likely depends on the particular risk factor. These you mentioned often result in some non-recoverable tissue damage before they are brought under control which might still leave a person more susceptible to Covid-19.

For example, hypertension can damage vessels and so even when the pressure is brought down, damage can remain.

OTOH, one of the larger risk factors for Covid-19 death is obesity. Losing weight will reduce that substantially.

 

[PeterNSteinmetz]

Is that correct? Even if the vaccine does not make one immune, it could certainly reduce the risk of infection by a vaccinated person by reducing the amount of replication-competent virus, reducing the period of infectiousness, and the severity of new infections-- all of with would bring down the r0. Would that not change the total number necessary for effective herd immunity?

Yes. See post #53 above for the formula. These are quantitative questions.

Unfortunately difficult for many of the general public to understand as they like to think in qualitative terms like “If I am vaccinated, I am immune and cannot spread COVID-19”. That kind of thinking works fairly well when the numbers are like 99.5%. Not so well for numbers more in the middling range. Leads to a lot of the current acrimony.

 

[PeterNSteinmetz]

It's interesting that your point is that these are quantitative questions, because that is exactly the point that I was asking about. The statement I was asking about was: "infections required for herd immunity remains the same with/without the vaccine." Would not the number of infections required for herd immunity be decreased if the vaccine reduces the amount of replication-competent virus, thereby reducing the period of infectiousness, and the severity of new infections? My question assumes the premise that vaccination does not completely prevent the spread of Covid.

Yes, I think you are correct and the statement you have here in quotes is incorrect.

If the amount of replication competent virus is reduced, that could well reduce the effective reproduction number, thereby changing the fraction required for herd immunity, roughly by the formula in post #53.

OTOH, I don’t think there is hard evidence this has happened.

 

[PeterNSteinmetz]

Why would a mathematician know anything more about public health, epidemiology, or virology than your average Joe? Mathematical formulas can also describe how water moves through my pipes, but if I my pipes are leaking, I'm going to call a plumber, not a mathematician.

The average Joe does not think nor reason quantitatively. They have a hard time with things that have to be thought about it quantitative terms, like reducing your odds of infection by a given percentage. They like to think qualitatively, like “If I get vaccinated I can’t catch Covid-19”.

Mathematicians and others with quantitative training are usually more comfortable with thinking in terms of odds.

 

[PeterNSteinmetz]

True, but it's also a case of "a little learning" (is a dangerous thing). They know just enough to be overconfident, but not enough really to know what they're talking about when they set up those YouTube channels.

Have you heard the joke about the dairy company that hired a physicist which ends with “consider a spherical cow?”