Thread by @michaelmina_lab

🧵 Why some vaccines stop transmission & others don’t:

Vaccines that stop transmission are often against viruses that replicate internally first and only then transmit (like measles). So if the vaccine stops internal replication, it stops transmission.

For COVID vaccines…

1/

For vax against upper respiratory viruses like SARS2, these viruses often don’t require “internal” replication. They just land in the nose, replicate locally & transmit on. So the vax can block “internal” replication and thus stop disease separate from stopping transmission

2/

The immune response in the upper respiratory tract is somewhat distinct from response in lungs, blood, lymph, etc.

Most vaccines create a multitude of layers of protection that can block a pathogen in lungs, blood, lymph, without offering the same in the nose/mouth/throat

3/

This is by no means a complete discourse on immunity and differences in transmission vs disease. But this is one reason why a vaccine against a virus like measles will stop spread while a vaccine against SARS2 or flu may not do so as well.

4/

Also

Above, I was trying to write something that ppl with no science background could understand.

I meant that local replication (for SARS2, within ACE2 expressing cells) is what is required for onward spread without systemic infection as a pre-requisite for onward spread.