Flu could cause similar symptoms to Covid in the long term, a study suggests.
Patients admitted to hospital with seasonal flu or Covid-19 were both at greater risk of death over the following 18 months.
Breathing problems were the most common, with patients more likely to be readmitted to hospital if they had suffered from either condition.
Experts say this shows the importance of annual jabs, especially among the elderly and the most vulnerable.
An important lesson we have learned from Covid is that an infection initially thought to cause only a short illness can also lead to chronic illness.
Covid was initially thought to cause only a short illness, but it was discovered that it also led to chronic diseases. Researchers from the University of Washington looked to see if the same was true for the flu
This spurred researchers at Washington University School of Medicine to look at the long-term outcomes of Covid in addition to the flu.
They analyzed more than 90,000 data from patients up to 18 months after infection with both viruses, comparing the risks of death, hospitalizations and 94 adverse health outcomes involving the body's major organ systems.
The highest risk was 30 days after initial infection for both conditions, according to the findings published in the Lancet.
Although Covid presented a greater risk of health loss than seasonal flu, infection with both viruses carried a significant risk of disability and illness.
Covid patients had an increased risk of 68 percent of health problems examined across all organ systems, compared to 6 percent with flu – which mainly occurred in the respiratory tract.
Ziyad Al-Aly, a clinical epidemiologist at Washington University, said: 'Many people think they are over Covid or the flu after they are discharged from hospital.
'That may be true for some people. But our research shows that both viruses can cause long-term illness.'
He added: 'For both Covid and seasonal flu, vaccinations can help prevent serious illness and reduce the risk of hospital admissions and death.
'Optimizing vaccination coverage must remain a priority for governments and healthcare systems around the world. This is especially important for vulnerable populations, such as the elderly and those with weakened immune systems.”