Hong Kong, Jan 29 : About 71 per cent of people with long Covid required medical treatment or were unable to work for six months or more, according to a new study. Some 18 per cent of patients -- most of whom were younger than 60 -- had still not returned to work more than a year after their initial infection, according to the study by the largest workers' compensation fund in New York. At least 65 million people are thought to have long Covid, according to a latest review of 200 Covid-19 studies published in Nature Reviews Microbiology journal earlier this month. The US-based researchers estimate that the condition is present in 10 per cent of the 651 million Covid-19 cases that have been documented worldwide, reports South China Morning Post. The World Health Organisation (WHO) published a standardised clinical case definition of long Covid in 2021, but experts say diagnosis is still a challenge. Most Asian countries do not track the number of long Covid patients. Experts believe that the best protection from long Covid is vaccination. Another study this month warned that Covid-19 patients have an elevated risk of death for at least 18 months after they got infected. Covid patients were more likely to develop numerous cardiovascular conditions compared to uninfected participants, which may have contributed to their higher risks of death, according to the study of nearly 160,000 participants published in Cardiovascular Research, a journal of the European Society of Cardiology (ESC). "The findings indicate that patients with Covid-19 should be monitored for at least a year after recovering from the acute illness to diagnose cardiovascular complications of the infection, which form part of long Covid," said Professor Ian CK Wong of the University of Hong Kong, China. Compared to uninfected individuals, the likelihood of Covid-19 patients dying was up to 81 times higher in the first three weeks of infection and remained five times higher up to 18 months later. /IANS