Flu's getting scarier by the day. Three-quarters of people infected with seasonal flu and swine flu in recent years have shown no symptoms. In other words, in today's world you might be down with influenza till the end, but show no symptom of it. In any case, for instance, not everyone with flu will have fever.
Researchers analysed data gathered in England during the winter flu seasons between 2006 and 2011, including the 2009 H1N1 "swine flu" pandemic, and found that one in five of the population were infected in both outbreaks, but just 23 per cent of these infections caused symptoms, and only 17 per cent of people were ill enough to consult their doctor.
These findings come from a major new community-based study comparing the burden and severity of seasonal and pandemic influenza in England over five years, published in The Lancet Respiratory Medicine journal on Monday.
The study indicates that primary-care surveillance greatly underestimates the extent of infection and illness in communities. The rate of influenza across all winter seasons was on average 22 times higher than rates of disease recorded by the Royal College of General Practitioners Sentinel Influenza-Like Illness Surveillance Scheme.
“Reported cases of influenza represent the tip of a large clinical iceberg that is mainly invisible to national surveillance systems that only record cases seeking medical attention,” said lead author Dr Andrew Hayward from University College London.
Herein lies the danger. “Most people don’t go to the doctor when they have flu. Even when they do consult they are often not recognised as having influenza. Surveillance based on patients who consult greatly underestimates the number of community cases, which in turn can lead to overestimates of the proportion of cases who end up in hospital or die. Information on the community burden is therefore critical to inform future control and prevention programmes.”
According Hayward, “Despite its mild nature, the 2009 pandemic caused enormous international concern, expense, and disruption. We need to prepare for how to respond to both mild and severe pandemics. To do this we need more refined assessments of severity, including community studies to guide control measures early in the course of a pandemic and inform a proportionate response.”
The additional warning came in the form of a linked comment published along with the study. Dr Peter William Horby from the Oxford University Clinical Research Unit in Vietnam, wrote, “An important unanswered question is the extent to which mild and asymptomatic influenza infections contribute to transmission... A large number of well individuals mixing widely in the community might, even if only mildly infectious, make a substantial contribution to onward transmission.”
Dr H Paramesh, pulmonologist and medical superintendent with the city's Lakeside Hospital, said, "Most infections take time to develop and they start with just fever." Senior consultant Dr Ramana Rao said, "Flu can be diagnosed only if there are symptoms and if it interferes with one's day-to-day activities."
With inputs from Luna Dewan
THE BACKGROUND
Influenza causes roughly 250,000–500,000 deaths worldwide each year. In the 20th century, there were three influenza pandemics for which there are varying mortality estimates: 1918 at least 20–40 million excess deaths, 1957 about 4 million excess deaths, and 1968 about 2 million excess deaths. In 2009 a new pandemic virus, H1N1, emerged in Mexico and spread globally over 2009–10, causing an estimated 200,000 respiratory deaths and 83,000 cardiovascular deaths during the first 12 months of circulation.
THE STUDY
The study was funded by the Medical Research Council and the Wellcome Trust and is a collaboration between researchers at UCL, Public Health England, University of Oxford, University of Nottingham, London School of Hygiene and Tropical Medicine, and Imperial College London.
SO, WHAT'S INFLUENZA (ALSO CALLED FLU)?
Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccine each year.
SIGNS AND SYMPTOMS OF FLU
People who have the flu often feel some or all of these signs and symptoms:
- Fever or feeling feverish/chills
- Cough
- Sore throat
- Running or stuffy nose
- Muscle or body aches
- Headaches
- Fatigue
- Vomiting and diarrhoea (more common in children).
How flu spreads: Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze or talk.
Period of contagiousness: You may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Most healthy adults may be able to infect others beginning day 1 before symptoms develop and up to 5-7 days after becoming sick.
PREVENTION
Prevention: The most effective way to prevent the disease and/or severe outcomes from the illness is vaccination. WHO recommends annual vaccination for:
- pregnant women at any stage of pregnancy
- children aged 6 months to 5 years
- elderly individuals (≥65 years of age)
- individuals with chronic medical conditions
- health-care workers.
KEY FACTS ABOUT SEASONAL INFLUENZA
- Seasonal influenza is an acute viral infection that spreads easily from person to person.
- Seasonal influenza viruses circulate worldwide and can affect anybody in any age group.
- Seasonal influenza viruses cause annual epidemics that peak during winter in temperate regions.
- Seasonal influenza is a serious public health problem that causes severe illness and death in high risk populations.
- An influenza epidemic can take an economic toll through lost workforce productivity and strain health services.
- Influenza vaccination is the most effective way to prevent infection.
- Antiviral drugs are available for treatment, however influenza viruses can develop resistance to the drugs.
Sources: Centers for Disease Control and Prevention (CDC), US; World Health Organization (WHO)
