eli5: What does it mean when immunologists say we need more research on transmission?

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So the recent Moderna/Pfizer vaccine is said to be safe and effective. However, wouldn’t this entail that it prevents transmission?

Any help would be great!

In: Biology
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Maybe. It was a concern that the vaccines may inhibit the negative symptoms of the disease but could still allow vaccinated individuals to be essentially asymptomatic carriers. This was not thoroughly tested when the vaccines were initially rolled out, but studies have since shown that they do indeed reduce the viral load which mitigates transmission.

>However, wouldn’t this entail that it prevents transmission?

Probably, but not necessarily. Some vaccines, like [the influenza vaccine, don’t completely eliminate the ability to catch and spread the virus.](https://pubmed.ncbi.nlm.nih.gov/33268518/) You inhale some virus, it infects your nasal tissue, it starts growing and releasing virus into your breath- and then your immune system crushes it before you feel sick or have any other problems. But you still have a short window where you’re contagious. Many other vaccines make it completely impossible to spread virus. No one knows which pattern the new vaccine will be like, so we need more research.

Note that the study I linked to is in pigs, which are a good study subject, because they live in close confinement. And note that the vaccine still reduced the spread, even though it didn’t eliminate it completely.