Thread by @JonathanKBall

I don't know why I (and many colleagues) have to keep saying this but #COVID19 vaccines do not prevent infection and therefore do not prevent emergence of variants. Sub-sterilising immunity through infection or vaccination will drive emergence of variants.

Please don't use this line of argument to underpin vaccine equity statements. Vaccine equity is important. Very important. But it should not be argued that fairer vaccine access will protect the world from future infections or variants.

It is possible to generate sterilising immunity but incredibly difficult to maintain. Therefore, we need to accept that infections will continue in future, variants may well emerge.

Current vaccines won't prevent this. BUT they do prevent disease...

The tweet above should be read in context of related tweets about vaccine inequities and emergence of variants.

We know that some vaccines may generate sterilising immunity and prevent immunity. But for existing covid vaccines there is a body of evidence to show that reinfections can occur in a significant proportion of people, and this is most likely due to waning levels of circulating Ab

I.e., the ability to provide sterilising immunity is short-lived. That doesn't mean you are not immune from disease. Your memory population will protect you BUT it will take a few days to kick in.

During that interval the virus will replicate. As antibody levels rise, that replication will occur in the face of rising antibody (and cellular) immunity. This is true for immunity through natural infection or vaccination.

If some of the mutations provide antibody escape and that virus is transmitted to someone else lacking sterilising immunity but has some immunity then that will favour that variant.

Other variants will arise in immunocompromised people, possibly during long-term infection. It is unlikely that vaccines will long-term prevent this from happening

Is sterilising immunity possible. We saw it in many people after vaccines were rolled out. But long-term? Maybe, especially if the virus hasn't mutated.

So maybe emergence of vaccine escape variants can be controlled by vaccinating the global population. Yes, so long as everyone receives sufficient vaccine to provide sterilising immunity and we don't have partially immune populations to incubate variants.

We think 2 doses gave reasonable levels of sterilising immunity for a few months after second dose. So if we had suppressed virus spread in every country, whilst we rolled out and administered two doses of vaccine to nearly every living person in the world...

(or ~75% of the population if R0=3) then it was achievable. But was this feasible? I would argue that targeting vulnerable people to protect against disease would be more achievable.

So was providing boosters to less vulnerable people during the UK omicron wave a waste of time. I'm sure it reduced some infection and transmission, but we don't know how much - reduction in symptomatic infection was ~75%.

But I was a big fan of boosting vulnerable people. I was less certain about all adults and probably still am...
Other views always to be found on twitter...