Open questions of the pandemic, part 3

As we head into the third year of the novel coronavirus pandemic, I've been reviewing some questions that are still open. The last two posts can be found here and here.

Today's further questions are inspired by the following chart put out recently by the White House.

On the surface, this chart provides evidence that the Covid-19 vaccine may protect people from getting hospitalized. If you're serious about causal analysis, you'd say all it depicts is a correlation. For the above chart, which plots "real-world" data, cannot be interpreted as if the data came from an RCT. In other words, the two groups (vaccinated and unvaccinated) are not identical except for vaccination status.

6. Why are unvaccinated people being hospitalized at an accelerating rate?

Focus on the blue line for the time being. This line shows the hospitalization rate for unvaccinated people, expressed in number of hospital admissions per 100,000. Between start of July and end of November, 2021, the weekly hospital admissions of unvaccinated people exploded from 6 per 100,000 to 68 per 100,000. Those are rates not counts.

This seems strange to me since I'd have expected the VE to come from a sharply reduced rate in vaccinated people while the rate among unvaccinated remains steady.

During that same period, the proportion of Americans who became fully vaccinated grew from 50% to 60%. The vaccines are said to be extremely effective, which should lower the amount of virus circulating in the population. Further, the unvaccinated proportion shrank from 45% to 30%, which is a 33% reduction rate!

The same dynamic shows up on the deaths chart as well. In other words, as more and more Americans get two doses, and fewer and fewer remain unvaccinated, the death rate among the remaining unvaccinated has skyrocketed.

Why? Why? Why?

7. How come VE is increasing while immunity is waning?

The definition of vaccine effectiveness is the relative ratio of the event rates of vaccinated versus unvaccinated. Events may be cases, hospitalizations, deaths, etc. Disregarding many methodological issues that I frequently discuss on the blog, I'm taking their analysis at face value. VE when measured by hospitalizations is then the ratio of the two curves shown. What is very odd about the chart is that the value of VE fluctuates wildly from week to week.

As a reminder, this is the overall trend of hospital admissions in the U.S. in 2021:

Hospitalized cases tumbled to a seasonal low in July, and then started rising again. In the first half of the year, the media kept reiterating that the vaccines were directly responsible for the dropping hospitalizations. Later, they said lower immunity against Delta and/or waning immunity was the reason for the return of cases. This narrative implies that the VE would be highest in July and dropping as the cases rose in the second half of 2021.

Surprise, surprise. If we compute the ratios of the rates shown in the White House chart, VE in July was the lowest (87%) and as the hospitalization rate of unvaccinated jumped 10 times, VE nudged up from 87% to 94%.

Why? Why? Why?

***

Any seasoned data analyst can tell you the very first step to good data analysis is asking the right questions. In this current series of posts, I show that the media have not been asking some of the most difficult questions. We can't pretend that we understand what's going on, unless we confront these challenges head on.

Comments

[A Palaz]

So in I have read that some combinations of test segments and Simpson paradox may explain some of data patterns. Van you sees this possible?

[Kaiser]

AP: unfortunately, it's impossible to make further comments because there is no disclosure of the underlying model/data. The numbers are "age adjusted". This means that some underlying age distribution is assumed - in theory, the adjustment may remove any Simpson's paradox but since we don't have the model, we can only speculate.