Why is the Moderna booster a half shot?

Australia now has two different booster options to choose from when the inevitable time comes for the third dose of coronavirus vaccine.Moderna was approved by the country's vaccine expert group ATAGI over the weekend.We'll also be able to get the booster a month earlier, with the Government announcing a shorter period due to the threat of waning immunity and new variants.So if Moderna is a choice, should you take it over Pfizer? Does it even matter? And why is the Moderna booster only a half shot?Also on today's show:* A preprint study has found that Astra seems to wane faster than other vaccines, leaving no protective effect against symptomatic disease from Omicron 15 weeks after the second dose. What could this mean for the country's booster campaign?