Hugo de Jonge, health minister of The Netherlands, has indicated the country could be preparing another three coronavirus booster vaccines.

De Jonge wrote a letter to his country’s parliament Wednesday suggesting the Netherlands should consider additional rounds of booster vaccines to fight new variants, with two of these in 2022 and another shot in 2023.

The letter also said that the Netherlands has enough booster vaccines for its current booster campaign.

In the letter, De Jonge wrote: “Certainly because only half of a regular vaccine is needed for a booster dose of Moderna, we now have sufficient vaccines for the current booster campaign and there is ample basis for possible extra booster rounds in the second quarter and the autumn of 2022 and in 2023.

But other experts argued that not enough was known about the effects of a fourth shot, and some scientists have raised concerns that too many shots might cause a sort of immune system fatigue, compromising the body’s ability to fight the virus, particularly among older people.