Thread by @yaneerbaryam

Thread: Breakthrough infections

Article: Breakthrough cases are a bigger problem than you’ve heard.

Key point: Vaccination reduces the risk of Delta infections by 2X.

But what about severity? See also article, but let's look at important CDC data

1/

nymag.com/intelligencer/…

Key CDC data:

“percentages of fully vaccinated persons infected…were hospitalized (3.2%), were admitted to an intensive care unit (0.5%), and required mechanical ventilation (0.2%) compared with…unvaccinated (7.6%, 1.5%, and 0.5%, respectively)”

2/
cdc.gov/mmwr/volumes/7…

According to these numbers the ratio of unvaccinated to vaccinated
hospitalizations is 7.6%/3.2% = 2.4X,
ICU cases 1.5%/0.5% = 3X, and
ventilations 0.5%/0.2% = 2.5X.

So the ratios are about 2.5-3X reduction in severity due to vaccination.

3/

Most of the rest of the report gives raw figures, not the risk ratios that you want to know.

In summary, there are two different effects, the rate of infection reduction, and the rate of severity reduction once you have an infection.

4/

The reduction in infection rate is 10-20X pre Delta and 2X with Delta and declines by waning. The reduction in severity of infections that do occur according to above data is 2.5-3X

Combine these two for a reduction of severe cases by 5-6X. Important, but not a solution.

5/

There is more however.

Delta is more severe by about 2X, perhaps higher.
thelancet.com/journals/lanin…
medrxiv.org/content/10.110…
thelancet.com/journals/lance…
thelancet.com/journals/lanin…
ajog.org/action/showPdf…
(one study says no difference
medrxiv.org/content/10.110…)

6/

So Delta's increase in severity counters vaccination's reduction of severity to essentially a wash.

[For clarity: Without vaccination you get the 2X increase in severity. So vaccination does help.]

7/

Also, for infection rate, exposure over time matters. Risk reduction over a period of time does not mean that infection won’t happen if exposure continues. Exposure also depends on behavior.

8/

If a person reduces protection by a change in behavior, they increase their risk, countering the reduction from the vaccination. How big is that effect? It depends on the change in behavior and the role of individual differences.

9/

From reported superspreader events most people can be infected, so individual differences don’t matter much.

So increasing the number of exposures counters the 2X reduction from vaccination over the same period of time. And the risk over time continues to increase.

10/

So protection from infection is a matter of behavior choice and time, and severity of cases is what matters to the outcome of the infections that do happen. Since the severity is 2X higher for Delta and 2.5-3X lower by vaccination, there hasn’t been an improvement overall.

11/

Combining these two points, vaccinations continue to provide important protection, but the combined result of having Delta and vaccination doesn’t improve the situation over the original variant without vaccination. It may even make it worse depending on behavioral choices.

12/