Flublok
influenza vaccine, recombinant hemagglutinin · Influenza HAAntigen content of quadrivalent product matched licensed trivalent vaccine plus a second B strain; no separate dose-finding
The public FDA and EMA reviews of HBVaxPro don't say how the first human dose was chosen. That rationale is in the IND, which is confidential, so it is missing for most approved products.
Find the highest dose with no adverse effects (NOAEL) in each animal species, convert it to a human equivalent dose, usually by body surface area, pick the most appropriate species, then divide by a safety factor (default 10) to get the maximum recommended starting dose.
NOAEL / key findings: No toxicologically significant findings in rat; changes attributed to the vaccine properties of test and reference vaccines (EMA assessment report, p. 4)