Influenza A cases rise sharply
According to the National Children's Hospital, the number of children seeking treatment for influenza A rose from 106 in week 35 to 249 in week 37, 319 in week 38, 552 in week 39 and 779 in week 40.
Some developed severe complications requiring intensive care, with at least one death reported.
Elderly patients and those with underlying conditions are also at risk.
Dr Le Thi Huyen of the National Hospital for Tropical Diseases said a 72-year-old man with hypertension and a history of stroke was admitted on October 6 with high fever, coughing, breathing difficulties and respiratory failure.
Severe pneumonia impaired his lung function, requiring respiratory support with an oxygen concentration of 40-50 per cent.
After three days, his fever subsided and his condition stabilised somewhat, but he continued to require non-invasive ventilation because of extensive lung damage.
Another patient, aged 73, with a history of smoking and chronic obstructive pulmonary disease (COPD), developed severe respiratory failure after just four days of coughing and a mild fever of around 38 degrees Celsius.
He tested positive for influenza A and was transferred to the National Hospital for Tropical Diseases, where doctors intubated him and provided intensive care.
Despite treatment, his condition continued to deteriorate, with serious systemic complications.
Huyen said patients with a history of stroke may struggle to clear mucus from their airways, increasing the risk of complications when pneumonia develops.
Rare brain complications in children
Dr Le Nhat Cuong of the National Children's Hospital warned that influenza A can cause acute necrotising encephalopathy (ANE), a rare but potentially fatal neurological complication.
The condition, often associated with viral infections, is believed to result from an excessive immune response that can cause brain swelling, bleeding and tissue damage. Severe inflammation may also affect other organs.
ANE typically develops several days after symptoms such as fever, coughing or respiratory infection, particularly in children aged two to five.
Warning signs include sudden drowsiness, seizures, impaired consciousness, coma and muscle stiffness.
Cuong urged parents to seek medical attention if children develop seizures following a fever rather than assuming they are harmless fever-related convulsions.
Although most children recover from influenza, severe cases can cause pneumonia, myocarditis, encephalitis, respiratory failure or multiple organ damage.
Doctors advised immediate medical attention for persistent high fever, seizures, unusual drowsiness, rapid breathing or breathing difficulties.