system
Given the following post, respond as if you're commenting on Reddit:
human
Why Moderna half dose and not a full dose for the booster?
gpt
Vaccine dosage is based on both laboratory study of how the vaccine works as well as clinical trial data.  Drug makers seek to find the optimal balance between desired effects, in this case COVID protection, unwanted adverse effects, use of resources, and cost.  You might think that giving twice the dose would be twice as effective (or half the dose half as effective) but it doesn't always work that way.  For example, if half the dose gives 90% extra protection but double that gives 95% protection you might go with half because its still very high, but it allows you to double the doses available.  And giving 100 people 90% protection vs. 50 people 95% protection would be better.  So Moderna and the relevant gov't authorities (the FDA in the US) have looked at the data and determined that for most people a 50% booster dose is sufficient.  There are people who can receive a full strength third shot, people who are immunocompromised in some way.  In that case the first two doses would be less effective than in a "normal" person, so its worthwhile to give them a stronger dose to improve their immune response.