LISBON (Reuters) - Over half the new COVID-19 cases being reported in the Lisbon region are of the more infectious Delta coronavirus variant, preliminary data showed on Sunday as Portuguese authorities scramble to curb a worrying spike in infections.

Ricardo Jorge, the national health institute, said the Delta variant, first identified in India, represented over 60% of cases in the Lisbon area though still less than 15% in the northern half of Portugal.

The Alpha variant, which was previously dominant in Britain, is more prevalent across Portugal's north, representing 80% of infections there and only 30% in and around Lisbon, according to the institute.

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Portugal posted over 1,000 new COVID-19 cases for the fourth straight day on Saturday and the number of people testing newly positive every 24 hours is back to late February levels, when the country was still under lockdown.

However, about 2.5 million of Portugal's 10 million population has now been fully vaccinated against COVID-19. While there has been a slight increase in hospitalisations in recent days, there has been no noticeable rise in deaths given that most older, more vulnerable people have been vaccinated.

The recent jump in infections comes around a month after tourism-dependent Portugal opened to visitors from the rest pf the EU as well as Britain.

Most of the new daily infections were in the Lisbon area, where a weekend travel ban took force on Friday.

Portugal has the European Union's highest seven-day rolling average of cases per capita, according to data tracker ourworldindata.org.

A lockdown was imposed in January to tackle what was then the world's worst coronavirus surge that brought the health system to the brink of collapse. Most restrictions have since been lifted.

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Britain removed Portugal from its green list of foreign destinations on June 3, less than a week after a mass gathering of football fans in Porto during the Champions League final was allowed to take place.

Portugal said last week it would allow U.S. visitors into the country but they must show a negative COVID-19 test result on arrival.

(Reporting by Catarina Demony; Editing by Mark Heinrich)