By Bojan Pancevski, Drew Hinshaw and Jenny Strasburg

Europe's vaccine plans are unraveling amid a shortage of doses that is raising concerns about the continent's ability to immunize its most vulnerable populations and reopen its economy in the near term.

In the latest blow for the continent, advisors to the German government warned on Thursday that AstraZeneca PLC's vaccine shouldn't be given to people over the age of 64 because of a lack of data about its efficacy in this group.

The European Union's medicines agency is expected to recommend use of the company's vaccine on Friday. A recommendation from the agency not to use the shot in older people would upend national vaccination plans that are tilted toward protecting those most at risk of dying from the disease.

In a sign of the urgency spreading across the continent, German Chancellor Merkel will hold a national vaccination summit with representatives of manufacturers on Feb. 1 in an effort to resolve the shortage of vaccines, her office said late on Thursday.

Europe 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.

The EU, which centralizes vaccine procurement for its members, has ordered 2.3 billion doses from six manufacturers and set a target for the region's governments to immunize 70% of adults in the bloc by the summer, a goal that seems increasingly out of reach.

This means there is little hope of an end to lockdowns and other constraints on public life soon, spelling trouble for the region's economy, large parts of which depend on services, including travel and tourism, especially in the less-affluent south.

The shortage of doses has shut down vaccination centers across the region or restricted their activities, causing frustration among those who are theoretically allowed to get a shot but haven't been able to secure an appointment.

"I have old people who have always refused the vaccination against the flu, but now they are all calling. Some of them are really desperate. It's become something like a golden egg. You hunt for it," said Jana Bendova, a Slovak doctor.

The Slovak Health Ministry said its vaccine hotline received 500 calls a second after the country made people 74 and older eligible for a vaccine this month.

The delays began this month when Pfizer Inc. announced a temporary reduction in deliveries of the vaccine it developed with Germany's BioNTech SE, the first to be approved in the region, because of an upgrade the U.S. company was undertaking at its manufacturing plant in Belgium.

Then, the European Commission, the EU's executive body, began sparring with AstraZeneca after the company announced 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.

Manufacturers and politicians critical of the EU's procurement have pointed out that the Commission signed its purchasing contracts later than the U.S. and the U.K., is paying less for doses, and has generally taken longer to approve shots, putting it at the back of the distribution queue.

Europe was counting on the AstraZeneca shot to jolt its vaccination effort as the region 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 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. A spokeswoman for the institute confirmed the leak but refused to comment on it.

The guidance will be published on Friday, if and when the European Medicines Agency rules on the AstraZeneca vaccine. The German government would have to make a formal decision on withholding the vaccine from people over 65. In this case, it would likely give it to medical workers and people aged 60 to 65. Those over 60 make up more than a quarter of Germany's 83 million inhabitants, according to government data.

Germany is currently vaccinating people over 80, as well as nursing home residents and staff and medical workers who have close contacts with highly vulnerable patients. It has only vaccinated some two million people since Dec. 28, less than half the 5.7 million people over 80 in the country and about 2.4% of its population.

Many vaccination centers in the country have stopped giving appointments or cut their opening hours due to the shortage of vaccines.

"Given the shortage of vaccines, we have at least 10 hard weeks ahead of us," German 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.

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 antivaccine sentiment 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 to present data on whether the vaccine was effective in those groups.

France has administered 1.2 million vaccine doses since it began inoculations at the end of December. That is slightly more than half the doses it has received, mostly from Pfizer and BioNTech. France has also received tens of thousands of doses from Moderna Inc., which developed the other Covid-19 vaccine that has been authorized in the European Union.

France's mass vaccination campaign limped out of the starting gate, hamstrung by complicated procedures adopted to get the consent of nursing home residents and personnel, the country's first target population. The campaign accelerated when the government allowed all health-care workers and everyone 75 and older to get the shot.

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, Drew Hinshaw at drew.hinshaw@wsj.com and Jenny Strasburg at jenny.strasburg@wsj.com

(END) Dow Jones Newswires

01-28-21 1421ET