A modified version of AstraZeneca's COVID-19 vaccine tailored to combat a coronavirus variant first documented in South Africa could be ready by the end of 2021, an AstraZeneca official in Austria said in an interview published on Sunday.

Sarah Walters, AstraZeneca's Austria country manager, told the Kurier newspaper that studies, so far, indicating the existing AstraZeneca vaccine was less effective against the more infectious variant first documented in South Africa were "too small to draw final conclusions".

"In the meantime, AstraZeneca and Oxford University have started on modifications to the vaccine for the South African variant and we expect it will be ready by the end of the year, should it be needed," Walters told the Kurier.

Walters blamed challenges - including delivery delays for the AstraZeneca shot in the European Union - on the "complex process" of producing a vaccine, coupled with the extremely high demand arising from the coronavirus pandemic.

"We had to work without keeping a supply in reserve. As a result, we couldn't make up for unexpected events," she said. "We are confident that we will fulfill our commitment to deliver 300 million doses to the European Union this year."

The Kurier interview did not directly address ongoing investigations into health concerns over the AstraZeneca shot. The EU has put a warning label on the vaccine over its possible linkage to extremely rare blood clots, Denmark has completely halted use of the vaccine and Britain has advised people under 30 to get another brand of vaccine.

Asked about "thousands" of people in Austria who are cancelling their appointments for AstraZeneca shots, Walters said the company's plan was "to continue to transparently provide information about efficacy and safety to doctors, so that they can adequately inform people" of benefits and risks.

British and European Union medicine regulators have said that the overall benefits of using the vaccine outweigh any risks of rare clotting.