Fever, cough, fatigue, sore throat that persists, worsens or is unusual can be signs of co-infection with influenza and Covid-19, requiring testing to accurately determine.
Associate Professor, Dr. Pham Quang Thai, Deputy Head of the Department of Infectious Disease Control, Central Institute of Hygiene and Epidemiology, said as above, adding that in a context where seasonal flu can circulate year-round and Covid-19 still appears sporadically, people should not ignore familiar respiratory disease symptoms.
"A person can be infected with influenza virus and Sars-CoV-2 simultaneously, while the initial manifestations of the two diseases overlap significantly, making it difficult to distinguish based only on clinical features," Associate Professor, PhD explained.
Clarifying further, Associate Professor, Dr. Pham Quang Thai said that the symptoms of the two diseases are very similar, including fever, cough, fatigue, sore throat, body aches or difficulty breathing. Therefore, just looking at the symptoms, it is difficult for doctors to confirm for sure whether this person has flu, Covid-19 or co-infection. For people at risk of serious illness, when necessary, testing should be done to identify the agent, thereby providing appropriate monitoring and treatment.
"Co-infection is not a common situation, but in some people, especially high-risk groups, the disease can progress more complicatedly," Associate Professor said.
Influenza and Covid-19 belong to the group of respiratory infectious diseases but are caused by two different groups of viruses: influenza is caused by influenza virus (influenza), while the cause of Covid-19 is Sars-CoV-2. The rate of co-infection depends on time, area, individual and testing strategy.
According to a report from the Hanoi Department of Health, by the end of June, there were still Covid-19 cases, but no deaths had been recorded and the cumulative number of cases was lower than the same period in 2025. These data do not show an unusual wave, but in fact, Sars-CoV-2 has not disappeared from the community.
Regarding the possibility of co-infection, a meta-analysis of 95 studies by a group of Chinese authors from 62,107 patients published on NCBI in early 2023 estimated the rate of influenza co-infection in people with Covid-19 to be about 2.45%. A study by Monireh Golpou and the author group on NCBI in 2025 estimated 14%, but the level of heterogeneity between studies is very high. Therefore, a single number should not be understood as a common rate for all communities.
According to experts, not everyone infected with flu and Covid-19 will have severe illness. The risk depends on age, underlying medical conditions, immune status, vaccination history, and when the patient accessed medical care. However, in high-risk cases, having to deal with two respiratory agents at the same time can make the disease more difficult to control.
Associate Professor Dr. said that the two viruses can cause inflammation and damage to the respiratory tract, making the immune response and the care process more complicated. In a 2023 NCBI study, a group of Chinese authors noted that co-infection with Sars-CoV-2 and influenza A had a 2.2 times higher risk of requiring intensive resuscitation treatment, 2.73 times higher risk of mechanical ventilation support, and even 2.92 times higher risk of death compared to people with only Covid-19.
According to experts, the older you get, the weaker your immune response becomes, while accompanying chronic diseases are more common, increasing the risk of severe disease progression.
Associate Professor Dr. Pham Quang Thai advises groups with underlying diseases such as cardiovascular disease, diabetes, bronchial asthma, chronic obstructive pulmonary disease, kidney disease, cancer or immunodeficiency... to be especially cautious. Respiratory tract infections can destabilize the underlying disease, increasing the risk of hospitalization.
Pregnant women face immune, respiratory and cardiovascular changes during pregnancy, which can make the flu more severe. Young children, especially under 5 years old, need to be closely monitored because their ability to compensate is limited when they have respiratory diseases.
In fact, both WHO and CDC (USA) have identified the elderly, young children, pregnant women, immunocompromised people and people with certain underlying diseases... as groups at high risk of complications from respiratory diseases. Experts recommend that early examination when there are unusual symptoms plays an important role.
Annual flu vaccination is one of the proactive measures to reduce the risk of flu, especially severe flu, complications and hospitalization. CDC emphasizes that the flu vaccine does not prevent Covid-19 and vice versa, the Covid-19 vaccine does not replace the flu vaccine. Reducing the risk of flu also reduces the chance of co-infection with flu and Covid-19.
In addition to vaccination, people need to maintain a healthy lifestyle by forming the habit of washing hands regularly, covering the mouth when coughing or sneezing, keeping the space well-ventilated, wearing a mask when having symptoms and limiting close contact with others during illness.
In addition, high-risk groups should contact a medical facility early if symptoms appear, because some antiviral drugs are more effective when prescribed early. You should see a doctor immediately if you have difficulty breathing, pain, persistent chest tightness, drowsiness, cyanosis, dehydration or symptoms gradually worsen.
