By Bojan Pancevski, Drew Hinshaw and Jenny Strasbourg

AstraZeneca PLC's vaccine shouldn't be given to people over the age of 64 due to a lack of data about its efficacy in this group, advisers to the German government warned on Thursday, further complicating Europe's stalled Covid-19 vaccine rollout.

The warning, a day before the European Union's medicines agency is expected to recommend use of the company's vaccine, comes as vaccinations are slowing across the region amid a shortage of doses that is threatening plans by European nations to immunize a large part of their populations by the summer.

In the latest setback for the bloc, the European Commission, the EU's executive body, has been sparring with AstraZeneca over the company's announcement last week that it may only be able to deliver as few as 30 million of the 80 million doses it pledged in the first quarter of 2021 due to manufacturing problems at a contractor's plant in Belgium.

The Commission has since accused the British-Swedish company of shipping doses made in the EU to the U.K. It has said it would ask drugmakers to notify authorities before exporting doses out of the EU. On Thursday, an EU official told reporters that they would introduce new criteria allowing national governments to block such exports.

AstraZeneca said it hasn't diverted any European supply to countries outside the EU.

Europe was counting on the AstraZeneca shot to relieve a shortage of vaccines in a region that is registering some of the world's highest daily rates of deaths and new cases. Increasingly, European officials are worried the shot will do little to ease those woes.

The continent is already lagging behind the U.K. and the U.S., which have vaccinated 11.3% and 7.1% of their populations respectively, according to data from Oxford University. By comparison, the EU has vaccinated just 2.2 % of its 450 million people.

One reason for the delays was a temporary cut in vaccine deliveries by Pfizer Inc. and BioNTech SE, makers of the first vaccine to be approved in the region, because of an upgrade to the U.S. company's plant in Belgium. But the European Commission, which centralizes vaccine procurement for the EU, also signed its purchasing contracts later than the U.S. and the U.K. and has generally taken longer to authorize shots.

The standing vaccination committee of Germany's Robert Koch Institute for infectious diseases said in an advisory note to the government leaked on Thursday that the AstraZeneca vaccine should be given only to people aged 18 to 64.

Germany is currently vaccinating people over 80, as well as nursing home residents and staff and medical workers who have close contact with highly vulnerable patients. The country has only vaccinated some two million people since Dec. 28, less than half the 5.7 million people over 80 in the country, and many vaccination centers have stopped giving appointments or reduced their opening hours due to the shortage of vaccine.

A spokeswoman for the RKI confirmed the content of the leak but refused to comment on it. The guidance will be officially published on Friday, when the European Medicines Agency is expected to rule on the AstraZeneca vaccine.

The German government would have to make a formal decision on withholding the vaccine from people over 65 once it is approved by the EU. In this case, it would likely give the shot to medical workers and people aged 60 to 65. People aged 60 and over make up more than a quarter of Germany's 83 million inhabitants, according to government data.

"Given the shortage of vaccines, we have at least 10 hard weeks ahead of us," Health Minister Jens Spahn wrote in a tweet on Thursday.

German Chancellor Angela Merkel said this week every adult in the country who wants a vaccine would be able to get one by the end of the summer as long as manufacturers delivered on their promises. Two thirds of the doses the Commission said it had ordered are for vaccines that have yet to be approved, which for some is only expected to happen late in the year.

The AstraZeneca vaccine, which is already in use in Britain, was developed together with Oxford University, but human trial data documenting the vaccine's efficacy in people 65 and older has been sparse so far, according to independent experts.

In December, peer-reviewed efficacy results published in the Lancet medical journal showed the vaccine to be safe and effective, but independent researchers said not enough trial results in the elderly were available to draw statistically significant conclusions. The University of Oxford and AstraZeneca said at the time they expected more data to become available as trials progressed. They said elderly volunteers were enrolled later in the trials as a safety measure.

AstraZeneca executives argued that the vaccine had already proven safe in the elderly, and that countries need to vaccinate all age groups, from adolescence to the oldest adults, to dent the pandemic globally.

Since Tuesday, the EU's drug regulator has been holding discussions over whether to recommend the vaccine for use in the bloc's 27 countries. The agency has said that given the scale of the pandemic, it would be willing to approve a vaccine that showed even modest effectiveness in preventing Covid-19.

But the agency is also worried about the skepticism towards a vaccine and has asked drugmakers to make sure that at least 7,500 test subjects in their clinical trials were older than 65, or had serious health issues. AstraZeneca has struggled to recruit elderly volunteers and present data on whether the vaccine was effective in those groups.

Italy planned to vaccinate 45% of its population in the first half of the year, starting with medical workers, nursing home staff and residents and those aged 80 and older. Now it says delays in vaccine deliveries could push these plans back by four weeks to eight weeks depending on the age group.

Spain is sticking to its goal of vaccinating 70% of the population by this summer and said on Thursday it was confident manufacturers could recoup the current delays, something experts see as optimistic.

"It's very hard to set goals when there are variables that you can't control, like the regular delivery of the vaccines by the pharmaceutical companies," said Amós García, a Spanish epidemiologist and president of the Spanish Association of Vaccinology.

--Giovanni Legorano in Rome contributed to this article.

Write to Bojan Pancevski at bojan.pancevski@wsj.com and Drew Hinshaw at drew.hinshaw@wsj.com

(END) Dow Jones Newswires

01-28-21 1205ET