Thread by @DennisZeilstra

I'm pleased to announce that my analysis of Israeli Covid data is published as a preprint. It's about vaccine effectiveness (VE) for infection. It assesses whether infection is more likely in unvaccinated people than in fully vaccinated people:
zenodo.org/record/5527285…

🧵

1/16

So what is vaccine effectiveness (VE)?
VE looks at the ratio of events in fully vaccinated (FV) individuals as percentage of the population at risk in that group (i.e. total No. of fully vaccinated people) to the same in unvaccinated (UV) people.

2/16

Suppose there are 10 events in a total of 100 FV people and there are 40 events in a total of 50 UV people, then VE is (100% x (1-(10/100)/(40/50)=) 87.5%. In this example the percentage of events in FV is much lower than the percentage in UV, thus VE is high.

3/16

Events can mean several things, such as infections (i.e. positive tests), hospitalization, or death. I assessed all of these.

When VE drops to 0%, the percentage of events in both groups is equal. In other words, if VE is 0% FV are equally likely to have an event as UV.

4/16

Thus, if VE for infections is 0%, fully vaccinated individuals are equally likely to test positive as unvaccinated persons. This would be evidence against a policy to test unvaccinated but not fully vaccinated people for e.g. e corona passport.

5/16

Why is this important? Because previous evidence only showed that fully vaccinated have a similar viral load as unvaccinated *if* they are infected. It is often argued that the risk of getting infected in the first place is lower. But is it?

6/16

In my analysis I've looked at Israeli data from 20 December 2020 till 4 September 2021 per age group (0-19, 20-29, ... ,80-89, 90+) per week. Data is available for vaccination, positive tests, hospitalization, and death. The latter 3 are stratified for vaccination status.

7/16

The data shows that in the first months, until mid-June 2021, the VE for infection is >90% in all age groups. After that, it rapidly drops in many age groups, in particular for 20-69. Other age groups start to decline a little later.

8/16

Now, what has happened in June 2021? The Delta variant happened.

The Delta rapidly spread and if you look at the percentage of the total population in each age group (both FV and UV), it increased exponentially *at an equal rate* in each group.

9/16

This is striking, because vaccination coverage in the various age groups is not the same. Yet, the infections are equally distributed among the complete Israeli population.

While VE was negative for some age groups in 2nd half of June, this is numerical noise. But...

10/16

...from mid-July until mid-August 2021 there were high numbers of positive tests in each age group. In that period, VE for infection in most age groups was around 0%. So FV tested positive equally often as UV.

11/16

The VE for infection started to increase after mid-August. Why? Apparently because of the booster doses. These were provided in the oldest age groups first, and the percentage of positive tests started lowering in the same order.

12/16

Although the VE for hospitalization and death did show some drop, they remained high throughout the entire period.

13/16

So, it can be concluded that after approximately 6 months, the VE for infection drops and fully vaccinated tested positive equally often as unvaccinated. This does not seem to be due to the Delta but to time: the booster (same as first doses) helped to increase VE again.

14/16

Together with the evidence that the viral load in breakthrough infections in fully vaccinated is as high as in first infections of unvaccinated people, this evidence shows that testing unvaccinated but not fully vaccinated is a policy not backed by evidence.

15/16

I invite everyone to read the full text and I welcome all substantiated comments.
zenodo.org/record/5527285…

I also encourage all countries to make available the Covid data as complete as Israel does. This helps to gain understanding and stimulates open science.

16/16

BONUS: because I am not a virologist, I did not include any speculation about what may cause this in the manuscript. I do want to make a suggestion here. It is known that antibodies (ABs) are waning after ~6m, but memory cells are not. It seems to make sense that when ABs....

... disappear, fully vaccinated become susceptible to infection and the virus is not immediately cleared. Hence, the virus can replicate and people become infectious. However, because memory cells are doing their thing, they are less likely to get severely ill. With a booster...

...the antibodies are (temporarily) boosted as well and the virus can again be rapidly cleared. So, after the booster dose the VE for infection goes up again. The question is how long lasting this will be and there simply is no data to answer this.