The European Medicines Agency's endorsement comes weeks after it began evaluating extending use of the vaccine, developed with Germany's BioNTech, to include 12- to 15-year-olds. It is already being used in the European Union for those aged 16 and older.
Inoculating children and young people is considered a critical step toward reaching "herd immunity" and taming the pandemic.
Scientists have been studying the benefits of several inflammation-fighting medicines to treat COVID-19, including some arthritis treatments and corticosteroids such as dexamethasone and budesonide.
An infection of the novel coronavirus can cause swelling in organs, including the lungs, which can also lead to severe complications.
"It's an extremely rare event. We hope by making people aware as well as putting clear diagnostic and therapeutic guidance in place that we can restore the confidence in our vaccine," said J&J's Chief Scientific Officer Paul Stoffels.
J&J, during a call to discuss its first-quarter results, said it would fulfill its commitments to ship 200 million doses in Europe and 100 million in the United States.
The European Medicines Agency (EMA) said its safety committee concluded that a warning about unusual blood clots with low blood platelets should be added to the vaccine's labels.
The EMA found that all instances had occurred in adults under 60 years, mostly women, within three weeks of vaccination, adding that all available evidence, including eight reports of cases in the United States, were part of its assessment.
Austria has been in talks with Russia to buy a million doses of the vaccine, and Kurz said on March 31 the order would probably be placed the following week. However, that order has yet to be announced.
Kurz had recently avoided saying whether his country would await EMA approval of the vaccine, which has been used in the European Union only by Hungary so far. EMA has launched a rolling review of Sputnik V.
U.S. federal health agencies on Tuesday recommended pausing the use of Johnson & Johnson's COVID-19 vaccine after six women under 50 given the shot developed rare blood clots.
Cases of the virus have been increasing for several weeks in Spain, which has fully vaccinated some 3.02 million people and administered a total of 9.8 million shots.
"We expect this announcement will have a direct and immediate impact not only on our national vaccination plans, but also in our citizens' trust in vaccines against COVID-19," the letter said.
The EU is grappling with a slow vaccine rollout caused by supply problems and by repeated changes in the use of the AstraZeneca shot, which have increased vaccine hesitancy.
"We have decided to continue the pause and make a new decision by April 15," FHI chief Camilla Stoltenberg told broadcaster NRK.
"We have started several processes to map out whether there is a causality and to have a better basis to establish the real risk and a cost-benefit analysis for different age groups. To get more knowledge, it is necessary to have more time."
The vaccine is of great use for the elderly, among whom many fall seriously ill each day.
"At the same time, we haven't seen these rare and serious side effects among our elderly. That is the background to why we are lifting the suspension for people older than 65."
Washington has handed control of the funds to Guaido who plans to use it to pay for access to the Covax mechanism and to invest in the cold chain necessary to vaccinate against Covid-19.
The suspensions have provoked intense debate over whether it was prudent to put AstraZeneca inoculations on hold just as vaccination campaigns were beginning to gather pace.
"We do not know if the cases are linked to the vaccine," Sigurd Hortemo, a senior doctor at the Norwegian Medicines Agency told a news conference held jointly with the Norwegian Institute of Public Health.
The European medicine regulator EMA would investigate the three incidents, Hortemo added.