Low immunity against Covid-19 and a growing population of frail elderly is driving a surge in coronavirus deaths in Japan which had, for a long time, upheld some of the strictest pandemic restrictions.

Japan once boasted one of the lowest Covid mortality rates, but the figure has been trending upwards since the end of 2022.

It hit an all-time high on 20 January this year, surpassing the UK, US and South Korea, according to Oxford University's Our World in Data.

Japan was largely closed to foreign visitors from 2020 till mid-June last year. It opened its borders cautiously - at first, travellers had to be part of a package tour, buy medical insurance, and be masked in all public places.

Some schoolchildren had meals in silence for over two years as schools imposed bans on lunchtime conversations.

As restrictions are eased, however, the population's low Covid immunity may be causing infections to spike, local health experts told the BBC.

Most of the latest Covid fatalities are elderly people with underlying medical conditions, experts said. This contrasts with the initial spate of deaths that were due to pneumonia and were often treated in intensive care.

"It is also difficult to prevent these deaths by treatment," says Hitoshi Oshitani, one of Japan's leading virologists, adding that Covid was only the trigger.

"Due to the emergence of immune-escaping variants and sub-variants and the waning of immunity, it is getting more difficult to prevent infections," he says.

"Immune escape" is when the human host's immune system becomes incapable of responding against an infectious agent. New versions of the Omicron variant are known to be masters of immune evasion.

Before the Omicron variant struck, Covid deaths mostly occurred in cities like Tokyo and Osaka, but there are now cases across the country, said Dr Oshitani, who was once regional adviser to the WHO on communicable disease surveillance and response.

"In smaller prefectures and rural areas, the proportion of the elderly population is even higher than the national average. This changing geographic pattern may also contribute to the increasing trend of deaths," he said.

Japan is the oldest society in the world by various measures, and its share of elderly people has been increasing every year since 1950.

Elderly people who are getting infected in nursing homes or in community clusters are not receiving prompt treatment, says epidemiologist Kenji Shibuya, a director at the Foundation for Tokyo Policy Research.

Faster treatment can help, he says, but because of Japan's classification of Covid as a Class 2 or "very dangerous" illness, only government-designated hospitals can treat the infected. And they have been overwhelmed by the surging caseload.

Dr Shibuya has called for Covid to be downgraded and treated as a form of influenza, allowing all clinics and hospitals to treat patients who have the virus.

Prime Minister Fumio Kishida announced earlier this month that the classification would be lowered, but only on 8 May. Experts, including Japan's top coronavirus adviser Shigeru Omi, have been calling for this since last year.

Dr Oshitani and Dr Shibuya also say that the death rate could have been inflated by under-reporting of Covid cases due to asymptomatic infections and tweaks to physicians' reporting requirements last year. That said, Japan is one of few countries still providing daily Covid tallies.

Yasuharu Tokuda, a physician at the Health and Global Policy Institute, noted that the Japanese population's natural immunity - acquired through infection - had been low before the middle of last year.

He says natural immunity is stronger than that obtained from vaccination - and so low infection rates have led to low immunity in Japan, which in turn is causing more deaths.

Dr Oshitani pointed to a similar phenomenon in Australia, where the Covid death rate has been creeping up since it reopened borders early in 2022 after keeping them shut for two years.

Experts are divided on the trajectory of Covid in Japan. Dr Tokuda, for instance, believes future rates of infection and death will be lower.

Dr Oshitani, on the other hand, expects a bigger surge in deaths in the months ahead as affordable antiviral drugs are still not widely available.