Low vaccination rates against the latest versions of COVID-19 and influenza are putting pressure on health care systems, leading public health officials say.

In the United States, several European countries and other parts of the world, there have been reports of rising admissions to hospitals linked to respiratory infections in recent weeks.

Death rates have also ticked up among older adults in some regions, but are far below the COVID-19 pandemic peak.

Spain's government has reinstated mask-wearing requirements at health care facilities, as have some US hospital networks.

"Too many people are in need of serious medical care for flu, for COVID, when we can prevent it," said Maria Van Kerkhove, the World Health Organisation's interim director of epidemic and pandemic preparedness.

She cited "incredibly low" vaccination rates against flu and COVID-19 in many countries this season, as the world tries to move past the pandemic and its restrictions.

Governments have struggled to communicate the risks still posed by COVID and the benefits of vaccination since a global public health emergency was declared over in May 2023, infectious disease experts and health officials said.

Only 19.4 per cent of US adults have received this season's COVID vaccine based on the US Centers for Disease Control (CDC) and Prevention's National Immunisation Survey, despite a recommendation that all adults get an updated shot to protect against serious illness.

That compares roughly with 17 per cent of adults who got the bivalent booster in the 2022-2023 season, based on actual vaccine data reported to the CDC by states.

Almost half of US adults over 18 got a flu shot this season (44.9 per cent), roughly the same as last year (44 per cent), according to the CDC.

"We don't think enough people have gotten the updated COVID vaccine," CDC director Mandy Cohen said in an interview.

"Folks still aren't understanding that COVID is still a more severe disease than flu."

Flu represented 5.2 per cent of US emergency visits compared with 3.0 per cent for COVID in the week ended December 30.

Yet COVID accounted for 10.5 out of 100,000 admissions to hospital in that time, compared with 6.1 per 100,000 for flu.

Most of the updated shots being used in the US and European Union are made by Pfizer with German partner BioNTech or Moderna

In Europe, flu is circulating at a higher rate than COVID, the European Centre for Disease Prevention and Control (ECDC) said.

In total, 24 per cent of a representative sample of tests came back positive in the last week of 2023, up from 19 per cent a fortnight earlier.

The rates are in line with previous flu seasons, said ECDC's respiratory virus expert Edoardo Colzani.

But "now we have COVID-19 as a new, unwanted guest", he said.

The ECDC did not have vaccination rates for the continent for flu or COVID, but Colzani said early data showed COVID vaccine uptake well below pandemic levels.

In Europe, the new COVID shots are recommended for high-risk groups only, such as seniors and the immunocompromised.

Among these groups, the WHO says there should be 100 per cent coverage.

COVID rates are also rising in the southern hemisphere during their summer, the WHO said, because it is not yet a seasonal virus.

Last month, 850,000 new COVID cases and 118,000 new admissions to hospital were reported globally, a rise from November of 52 per cent and 23 per cent, respectively, according to WHO, which added that actual figures were likely higher.

The vaccines are still very effective at preventing serious illness, even if they do not block infection, experts said.

A recent study in the Lancet Infectious Diseases journal from the Karolinska Institutet and Danderyd Hospital in Sweden found the updated vaccine, which targets the XBB.1.5 coronavirus variant, reduced the risk of COVID admissions to hospitals by 76.1 per cent in people affected by more recent variants, based on public health records from adults 65 years and older.

This year's flu shots, made by a range of manufacturers, are estimated to reduce the risk of being admitted to hospital by 52 per cent.

But "fatigue for COVID vaccination" is hampering uptake, Colzani said.

In Italy, for example, 8.6 per cent of the eligible population have had their third COVID booster after the initial vaccination series, Ministry of Health data from January 7 showed.

The data for flu is not yet available, but a study by Federfarma, the association of Italian pharmacies, said 15 per cent of Italians had been vaccinated against flu this autumn, compared to some 20 per cent last season.