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Serbia will offer a third "booster" COVID-19 shot to its population from Tuesday, as the country battles an uptick in cases, largely among unvaccinated citizens.

The Balkan nation had a strong initial rollout, but rates dropped off after around 40 percent of the population was vaccinated, with many citing .

Daily virus cases are on the rise in Serbia, which epidemiologists have blamed in part on the highly-infectious Delta variant, largely striking the unvaccinated.

"After more than six months of mass vaccination in Serbia, the vast majority of hospitalised patients are unvaccinated citizens," the government said in a statement Monday.

That should "encourage those who have not yet been vaccinated to do so", it added.

Booster shots will be recommended for , including immunodeficient and elderly citizens, healthcare workers and frequent travellers.

But the government said anyone who received a second jab more than six months ago will be eligible to receive a third shot.

Citizens can choose which vaccine they would like to receive for their third shot, regardless of which jab they initially got.

Thanks to early deals with a range of pharmaceutical companies, Serbia acquired millions of from Russia, China and Western firms, though the majority were Chinese-made Sinopharm vaccines.

Since the beginning of the pandemic, the country of seven million has registered more than 7,100 deaths and over 732,000 coronavirus cases.