FRANKFURT -- German biotech firm CureVac said it has seen the number of requests for its experimental COVID-19 vaccine increase over the past few days, as concerns over rare side effects have hit some other coronavirus shots.

Remarks made to Reuters by a CureVac spokesman on the growing interest on Thursday were underscored by Chief Executive Franz-Werner Haas in an analyst call on fourth-quarter results.

"There is much more vaccine needed than is available. We are talking primarily with governments but also ... with COVAX," Haas said, referring to the international vaccine-sharing facility.

"There are very hot discussions," he added.

The European Union in November secured up to 405 million doses of CureVac's two-shot vaccine, which has yet to win regulatory approval, the company's only large supply contract so far.

Europe's choppy vaccine rollout hit more trouble this week after Johnson & Johnson (J&J) suspended shipments of its COVID-19 shot and Denmark said it would drop a similar vaccine from AstraZeneca over very rare cases of blood clotting.

In many European countries, AstraZeneca's shot has been limited to the elderly as the side effect in question, clotting in the brain, has primarily been seen in young and middle-aged vaccine recipients.

CureVac said in a statement on Thursday that it would start testing its COVID-19 vaccine on adolescents as young as 12 at the end of the month, as it gets ready to publish initial efficacy results for adults in the second quarter.

The spokesman reiterated the group - which is backed by investors Dietmar Hopp, the Gates Foundation, GlaxoSmithKline as well as the German government - hoped to file for regulatory approval at the end of May or early June.

CureVac said it would initially enroll about 40 adolescents aged 12 to 17 in Peru and Panama, the first stage of a broader study in this age group.

Its ongoing pivotal Phase 2b/3 study known as Herald, and initiated in December, has completed recruitment with over 40,000 adult participants in Europe and Latin America, the company said on Thursday.

Nasdaq-listed CureVac repeated it aim to produce up to 300 million doses of the vaccine in 2021 and up to 1 billion in 2022.

(Editing by Susan Fenton, Kirsten Donovan)