The EU's campaign to vaccinate its citizens against COVID-19 has got off to an uneven start in what will be a continued effort to administer enough shots to the bloc's 450 million people to defeat the viral pandemic.

The EU vaccination drive kicked off at the weekend, with health workers and residents of care homes across the bloc among the first to get the shots from Pfizer, which must be stored at ultra-cold temperatures.

For long-term storage, the Pfizer vaccine needs to be deep frozen at about minus 70 degrees Celsius (minus 94 degrees Fahrenheit). It can be defrosted for a few days before being used, but even then must be kept chilled at between 2 degrees Celsius (35.6 degrees Fahrenheit) and 8 degrees Celsius (46.4 degrees Fahrenheit).

In southern Germany, officials had to send back about 1,000 doses after finding they had been transported in cool boxes typically used for picnics or camping trips that failed to keep the vaccine cold enough.

"There were doubts as to whether the cold chain was maintained at all times,"said Christian Meissner, district administrator in the Bavarian town of Lichtenfels.

"BioNTech said that the vaccine was probably okay, but 'probably okay' is not enough," said Meissner.

In Spain, delivery of a new batch from Pfizer was held up by a day to Tuesday due to a temperature issue that has now been resolved, Health Minister Salvador Illa said.

Maria Asuncion Ojeda, a resident at Madrid's Ballesol Parque Almansa nursing home, was still delighted to be an early recipient of the Pfizer vaccine.

"I wanted to do it because it's the only way we can solve this problem," the 87-year-old said on Monday, a day after Spain began vaccinating care-home residents and their staff.

(With input from Reuters)