Influenza symptoms vary across populations

Infection with the H1N1 virus is virtually impossible to distinguish from infections caused by other influenza viruses. Symptoms of H1N1 infection are similar to most influenza infections, and include fever (100 F or greater), cough, nasal secretions, fatigue, and headache. Some patients may also get a sore throat, rash, body aches, headaches, chills, nausea, vomiting, and diarrhea. The incubation period from exposure to first symptoms is about one to four days, with an average of two days. The symptoms last about one to two weeks and can last longer if the person has a severe infection. Those with H1N1 infection appear to be more likely to have vomiting and diarrhea and H1N1 infections seem to affect younger people more than in the case of other influenza viruses. However these differences are not pronounced enough to make it possible to distinguish between H1N1 and other forms of influenza.

There is a huge variation in the severity of symptoms in those infected. It may range from minor nasal congestion and sore throat to much more severe symptoms of high fever, cough and pneumonia. Those who exhibit severe symptoms may also contract bacterial pneumonia. Most people with pneumonia infections will have mild to moderate symptoms, requiring no specific treatment. At the peak of the H1N1 pandemic, it was estimated in Mexico that about 6 per cent of those infected were dying – a very high figure. However by the end of 2010 the estimates had come down to just 1 in 1000 deaths in those infected (0.1 per cent). Subsequent studies led by the World Health Organisation (WHO) indicate that about 1 in 5,000 (0.02 per cent) of those infected with the virus died during the 2009 pandemic. This is much lower than in the 1918 "Spanish flu" pandemic, which had an estimated mortality rate ranging from 2 per cent-20 per cent. The discrepancies in estimates in 2009 and later are explained by the fact that initial estimates were based on those who actually tested positive for H1N1. Later, it was understood, that a much larger number were infected but were never tested. This was because most had such mild symptoms that they did not seek medical advice, or were not referred for specific testing for H1N1. A paper in the medical journal, the Lancet in 2014 noted that just 23 per cent of these who were infected by H1N1 in 2009 actually exhibited symptoms and only 17 per cent of people were ill enough to consult their doctor.

In fact, the WHO has been faulted for issuing a ‘level 6’ pandemic alert in 2009 – signaling acute emergency requiring drastic measures (and thereby contributing to widespread global panic). The WHO has been criticised for basing its perception of the pandemic on the geographic spread of the disease (which was clearly very extensive) but not accounting for the relatively mild severity of the disease.