BERLIN, Germany -- The first generation vaccine developed by BioNTech-Pfizer works against coronavirus variants such as the Delta strain and does not need to be modified for the moment, the chief executive of German company BioNTech said Monday.

"It is quite possible that in the next six to 12 months, further variants will emerge and that would require adaptation of the vaccine but it is at the moment not yet the case," Ugur Sahin told journalists.

A decision to make a switch should be made only if it is clear that the vaccine failed to work or is only offering sub-par protection against the virus.

The fast-changing situation meant that getting the timing for the change right was also crucial.

"Making a decision at the moment might turn out to be wrong in three or six months if another variant is dominating. Therefore the timing of the decision must be appropriate," he said.

"At the moment we have a good understanding that the booster vaccine with the parental strain is completely sufficient," stressed Sahin.

BioNTech's partner Pfizer has also repeatedly amplified the case for booster shots amid the latest wave of infections.

Countries including France and Germany have said they will begin offering the additional shot to the elderly and the most vulnerable from September.

BioNTech-Pfizer have shipped around one billion doses of their vaccines to more than 100 countries or territories around the world.

They are expecting their annual manufacturing capacity to reach three billion doses by year's end, before climbing to four billion doses in 2022.