In a joint statement released after a virtual summit on Friday, G7 leaders raised their overall commitment to $7.5bn (£5.3bn).
Wealthy countries are facing growing pressure to make sure lower-income nations get fair access to vaccines.
President Joe Biden has pledged $4bn in US aid to the fund.
An initial $2bn will be donated in 2021, with an additional $2bn coming over the next two years.
Germany pledged an additional $1.2bn with Chancellor Angela Merkel telling journalists: "I stressed in my intervention that the pandemic is not over until all people in the world have been vaccinated."
The Covax scheme is aiming to get at least 1.3bn vaccine doses to vulnerable populations worldwide in the coming months.
Covax is co-led by Gavi, known as the Vaccine Alliance, the World Health Organization (WHO) and the Coalition for Epidemic Preparedness Innovations (CEPI).
WHO chief Tedros Adhanom Ghebreyesus said on Friday he was grateful for the new commitments but emphasised that more was needed to address the world's vaccination imbalance.