The company said the reports were "completely incorrect".
AstraZeneca has defended its Covid-19 vaccine following reports in the German media, quoting unnamed sources in the German government, which expressed doubts over the vaccine's effectiveness among those aged over 65.
On Monday, the Handelsblatt economic daily reported the efficacy of the Oxford/AstraZeneca vaccine had been overestimated and was as low as 8% among the over 65s.
However, this claim has been rejected by the pharmacy giant who said the reports were "completely incorrect".
"Reports that the AstraZeneca/Oxford vaccine efficacy is as low as 8% in adults over 65 years are completely incorrect," the company said in a statement on Monday night.
"In November, we published data in The Lancet demonstrating that older adults showed strong immune responses to the vaccine, with 100% of older adults generating spike-specific antibodies after the second dose," it added.
The European Union issued an angry warning to AstraZeneca on Monday over its unexpected delay in delivering millions of doses of its Covid-19 vaccine to the EU.
AstraZeneca is believed to have received an upfront payment for at least 300 million doses when the bloc sealed a supply deal with the company in August.
"The answers of the company have not been satisfactory so far," Stella Kyriakides, the European Commissioner for Health and Food Safety, said in a statement.
"The European Union wants the ordered and pre-financed doses to be delivered as soon as possible. And we want our contract to be fully fulfilled."
The European Medicines Agency is expected to approve the vaccine for use in the European Union later this month.
The EU has currently authorised two vaccines for widespread distribution, manufactured by BioNTech/Pfizer and Moderna.
Unlike the Pfizer vaccine, the Oxford/AstraZeneca vaccine does not require temperatures of -70 degrees Celsius for storage and is seen as a more practical vaccine for widespread rollout, especially in poorer countries.
