Thread by @RajeevJayadevan

Natural infection provides greater protection than (Pfizer) vaccination

Large study from Israel compared 3 groups of people

1. Past infection
2. Pfizer-vaccinated individuals
3. Those who had both

Please see WHOLE thread👇

1/

https://twitter.com/MartinKulldorff/status/1430660291579105284

Large cohort of 673,676 vaccinated, 62883 past infection, & 42,099 vacc + past infection. The groups were matched to exclude confounding.

They looked at remote & recent past infection separately. Those who were infected in 2021 had greater protection than 1 year ago.

2/

The vaccinated group had a 27-fold greater risk of SYMPTOMATIC breakthrough infection compared to natural infection. The risk was 13-fold for ALL breakthrough infections.

A single dose of vaccine further increased the level of protection for those who had past infection.

3/

Those who had a recent infection had a 13-fold greater protection compared to vaccination.

Those who were infected a year ago still had better protection compared to vaccination; this was 7-fold.

The protection included symptomatic disease as well as hospitalisation.

4/

The study is a direct comparison between natural infection and vaccination.

🔺However this should not be misinterpreted that natural infection is “preferable” to vaccination.

That’s because the risk of complications with natural infection is far greater than vaccination.

5/

Conclusions.

Those who had natural infection can expect a greater protection against future COVID-19, including against hospitalisation.

Those with natural infection who had a single dose of vaccine enjoyed greater degree of protection.

However, some caveats 👇

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This does not mean or imply that those who are not vaccinated should “wait to be naturally infected” and forego vaccination.

🔺This is a potentially dangerous conclusion that could arise from the study, particularly if an older person decides to do that.

Here is why. 👇

7/

It was shown earlier (see earlier tweet) that age is the most significant risk factor for severe disease in COVID-19, far more than comorbidities- still a relatively unknown fact.

Thus, a 75-year-old with COVID-19 is at least 200 times more likely to die than a 25-year-old.

8/

Besides, the risk of complications of natural infection is far greater than that of vaccination.

For who are not yet naturally infected, vaccination is the best option.

A single dose of vaccine is definitely helpful for those with past infection (anamnestic response,👇)

9/

What is Anamnestic response?

If there was exposure to antigen in the past, a 2nd exposure (after a time-gap) will result in a hugely amplified immune response.

This is the reason why 2 doses of vaccines are given.

The same applies to vaccination after natural infection.

10/

Mechanism of anamnestic response:

Memory cells made following the 1st infection (or vaccine dose) are already primed & ready for a coordinated immune response by the time the 2nd dose arrives.

Memory cells work like amplifiers and also undergo ‘self-improvement’ over time.

11/

No study is without limitations. In this case, a potential confounder is the so-called Peltzman effect (vaccinated people might take more risks, socialise more without masks, and pick up more infections - while unvaccinated people might be more careful).

12/

The study did not address “boosters”. There is no evidence at this time that a 3rd dose will increase protection against severe disease.

3rd dose will transiently increase antibody level, but does not enhance memory cells. Thus, T cell immunity might not be “boosted”.

13/13

Vaccine effectiveness drops steeply past 3 1/2 months. But protection against severe disease is maintained.

https://twitter.com/RajeevJayadevan/status/1431509276581785603