To my knowledge no booster is approved for use within 6 months of another shot, and 6 months is overkill unless you're extremely high risk, e.g. in the "still wiping down door dash deliveries with bleach wipes" kind of high risk.
Government recommends booster after five to six months in my country. To clarify, they do not yet recommend boosters to the end of time, just a booster after the second.
> 6 months is overkill unless you're extremely high risk
It is a general recommendation here.
Three months, available to all adults.
"All adults in the United Kingdom will be able to get their booster Covid-19 vaccine doses three months after their second shot, the government indicated on Monday, in a dramatic acceleration of the country's inoculation drive that comes amid fears over the Omicron variant."