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There are around a billion cases of seasonal influenza annually, including 3-5 million cases of severe illness.

It causes 290 000 to 650 000 respiratory deaths annually.

Ninety-nine percent of deaths in children under 5 years of age with influenza-related lower respiratory tract infections are in developing countries.

Symptoms begin 1-4 days after infection and usually last around a week.

Overview

Seasonal influenza (the flu) is an acute respiratory infection caused by influenza viruses. It is common in all parts of the world. Most people recover without treatment.

Influenza spreads easily between people when they cough or sneeze. Vaccination is the best way to prevent the disease.

Symptoms of influenza include acute onset of fever, cough, sore throat, body aches and fatigue.

Treatment should aim to relieve symptoms. People with the flu should rest and drink plenty of liquids. Most people will recover on their own within a week. Medical care may be needed in severe cases and for people with risk factors.

Influenza A viruses are further classified into subtypes according to the combinations of the proteins on the surface of the virus. Currently circulating in humans are subtype A(H1N1) and A(H3N2) influenza viruses. The A(H1N1) is also written as A(H1N1)pdm09 as it caused the pandemic in 2009 and replaced the previous A(H1N1) virus which had circulated prior to 2009. Only influenza type A viruses are known to have caused pandemics.

Influenza B viruses are not classified into subtypes but can be broken down into lineages. Influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.

Influenza C virus is detected less frequently and usually causes mild infections, thus does not present public health importance.

Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

There are 4 types of influenza viruses, types A, B, C and D. Influenza A and B viruses circulate and cause seasonal epidemics of disease.

Signs and symptoms

Symptoms of influenza usually begin around 2 days after being infected by someone who has the virus.

sudden onset of fever

cough (usually dry)

headache

muscle and joint pain

severe malaise (feeling unwell)

sore throat

runny nose.

Symptoms include:

The cough can be severe and can last 2 weeks or more.

Most people recover from fever and other symptoms within a week without requiring medical attention. However, influenza can cause severe illness or death, especially in people at high risk.

Influenza can worsen symptoms of other chronic diseases. In severe cases influenza can lead to pneumonia and sepsis. People with other medical issues or who have severe symptoms should seek medical care.

Hospitalization and death due to influenza occur mainly among high-risk groups.

In industrialized countries most deaths associated with influenza occur among people aged 65 years or older (1).

The effects of seasonal influenza epidemics in developing countries are not fully known, but research estimates that 99% of deaths in children under 5 years of age with influenza related lower respiratory tract infections are in developing countries (2).

Epidemiology

People at greater risk of severe disease or complications when infected are pregnant women, children under 5 years of age, older people, individuals with chronic medical conditions (such as chronic cardiac, pulmonary, renal, metabolic, neurodevelopmental, liver or hematologic diseases) and individuals with immunosuppressive conditions/treatments (such as HIV, receiving chemotherapy or steroids, or malignancy).

Health and care workers are at high risk of acquiring influenza virus infection due to increased exposure to the patients, and of further spreading particularly to vulnerable individuals. Vaccination can protect health workers and the people around them.

All age groups can be affected but there are groups that are more at risk than others.

Epidemics can result in high levels of worker/school absenteeism and productivity losses. Clinics and hospitals can be overwhelmed during peak illness periods.

Transmission

Seasonal influenza spreads easily, with rapid transmission in crowded areas including schools and nursing homes. When an infected person coughs or sneezes, droplets containing viruses (infectious droplets) are dispersed into the air and can infect persons in close proximity. The virus can also be spread by hands contaminated with influenza viruses. To prevent transmission, people should cover their mouth and nose with a tissue when coughing and wash their hands regularly.

In temperate climates, seasonal epidemics occur mainly during winter, while in tropical regions, influenza may occur throughout the year, causing outbreaks more irregularly.

The time from infection to illness, known as the incubation period, is about 2 days, but ranges from 1-4 days.

Diagnosis

Most cases of human influenza are clinically diagnosed. However, during periods of low influenza activity or outside of epidemics situations, the infection of other respiratory viruses (e.g. SARS-CoV-2, rhinovirus, respiratory syncytial virus, parainfluenza and adenovirus) can also present as influenza-like illness (ILI), which makes the clinical differentiation of influenza from other pathogens difficult.

Collection of appropriate respiratory samples and the application of a laboratory diagnostic test is required to establish a definitive diagnosis. Proper collection, storage and transport of respiratory specimens is the essential first step for laboratory detection of influenza virus infections. Laboratory confirmation is commonly performed using direct antigen detection, virus isolation, or detection of influenza-specific RNA by reverse transcriptase-polymerase chain reaction (RT-PCR). Various guidance on the laboratory techniques is published and updated by WHO.

Rapid diagnostic tests are used in clinical settings, but they have lower sensitivity compared to RT-PCR methods and their reliability depends largely on the conditions under which they are used.

Treatment

Most people will recover from influenza on their own. People with severe symptoms or other medical conditions should seek medical care.

stay home to avoid infecting other people

rest

drink plenty of fluids

treat other symptoms such as fever

seek medical care if symptoms get worse.

People with mild symptoms should:

pregnant

children under 59 months of age

aged 65 years and older

living with other chronic illnesses

receiving chemotherapy

living with suppressed immune systems due to HIV or other conditions.

People at high risk or with severe symptoms should be treated with antiviral medications as soon as possible. They include people who are:

The WHO Global Influenza Surveillance and Response System (GISRS) monitors resistance to antivirals among circulating influenza viruses to provide timely evidence for national policies related to antiviral use.

Prevention

Vaccination is the best way to prevent influenza.

Safe and effective vaccines have been used for more than 60 years. Immunity from vaccination goes away over time so annual vaccination is recommended to protect against influenza.

The vaccine may be less effective in older people, but it will make the illness less severe and reduces the chance of complications and death.

Vaccination is especially important for people at high risk of influenza complications and their carers.

pregnant women

children aged 6 months to 5 years

people over age 65

people with chronic medical conditions

health workers.

Annual vaccination is recommended for:

wash and dry your hands regularly

cover your mouth and nose when coughing or sneezing

dispose of tissues correctly

stay home when feeling unwell

avoid close contact with sick people

avoid touching your eyes, nose or mouth.

Other ways to prevent influenza:

Vaccines

Vaccines are updated routinely with new vaccines developed that contain viruses that match those circulating. Several inactivated influenza vaccines and recombinant influenza vaccines are available in injectable form. Live attenuated influenza vaccines are available as a nasal spray.

WHO response

WHO, through the Global Influenza Programme and GISRS, in collaboration with other partners, continuously monitors influenza viruses and activity globally, recommends seasonal influenza vaccine compositions twice a year for the northern and southern hemisphere influenza seasons, guides countries in tropical and subtropical areas as to which formulation vaccines to use, supports decisions for timing of vaccination campaigns, and supports Member States to develop prevention and control strategies.

WHO works to strengthen national, regional and global influenza response capacities including diagnostics, antiviral susceptibility monitoring, disease surveillance and outbreak response, to increase vaccine coverage among high-risk groups, and to support research and development of new therapeutics and other countermeasures.

1. Estimates of US influenza-associated deaths made using four different methods.
Thompson WW, Weintraub E, Dhankhar P, Cheng OY, Brammer L, Meltzer MI, et al. Influenza Other Respi Viruses. 2009;3:37-49

2. Nair H, Abdullah Brooks W, Katz M et al. Lancet 2011; 378: 1917-3 Global burden of respiratory infections due to seasonal influenza in young children: a systematic review and meta-analysis.

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WHO fact sheet on influenza: includes key facts, definition, symptoms, transmission, seasonal epidemics, effects, prevention, WHO response.

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F or those who want to give vitamin C or zinc (or both) a shot, it's easy enough to find supplements of either. One popular zinc supplement, for example, is Zicam, which provides quick-dissolving tablets specifically marketed as shortening cold duration. Perhaps the best known (and reached for) product is Emergen-C, which provides 1000 milligrams of vitamin C in a packet of flavored powder which you dissolve in a glass of water. (There are other ingredients in Emergen-C, none of which have been convincingly linked to any cold-related benefits.)

But according to David Dildine, MD, an urgent-care physician with Atrius Health in Boston, there are no particular advantages or disadvantages to taking C and zinc via these sorts of products, other than it may be more convenient, and possibly more expensive, than other forms of the supplements.

And William Curry, MD, a professor of internal medicine at the University of Alabama at Birmingham, warns that any supplements, including vitamin C and zinc, can cause problems for some people if taken in high enough doses. If zinc is taken orally for much longer than a week or two, it can lead to a potentially dangerous shortage of copper in the body. "It's always worth talking to your doctor before taking any supplement," says Curry. "If an ingredient can provide some benefit, it can provide some risk of harm, too."

Taking more vitamin C than the recommended daily maximum of 2000 milligrams is generally believed to carry at least a mild risk of nausea and diarrhea. (Emergen-C has 1000 milligrams of vitamin C, so you should be fine.)

The best strategy for anyone hoping to improve their odds with colds, according to research, is to adopt the same healthy habits recommended for general health: Regular exercise, a healthy diet with plenty of vegetables, good stress management, and plenty of restful sleep.

None of these behaviors will do much for you if you engage in them after you've been hit with a cold. But they're all associated with at least a slightly lower risk of getting a cold in the first place, most likely because they can strengthen the immune system. "I see it all the time in the clinic," says Dildine. "People who don't take care of themselves, or who are stressed out, get colds and other illnesses more often."

Even if a supplement or remedy ultimately offers nothing but a placebo effect on a cold, that doesn't mean people should outright avoid it.

And there's one other behavior that can help fend off colds: Staying warm. Studies have shown that cold viruses tend to replicate more robustly at colder temperatures, and people exposed to colder temperatures are more likely to get colds. In animal studies, colder temperatures have also been shown to reduce the immune system's ability to fight off colds. Putting this insight to work probably requires staying inside more during cold weather - dressing more warmly outside wouldn't likely do the trick, because you'd still be breathing the colder air that seems to cause the problem. Of course, if staying inside ends up limiting your exercise or keeping you in closer contact with people who may have colds, you'd face a trade-off in risks.

As for fighting off a cold that's coming on, even if a supplement or remedy ultimately offers nothing but a placebo effect on a cold, that doesn't mean people should outright avoid it, says Dildine. "The placebo effect can be pretty powerful," he notes. "In studies, the placebo effect often comes out to be a 20% improvement in symptoms." Whether that's all in a patient's mind, or perhaps might be linked to a strengthening of the immune system as a result of a positive attitude remains an open question, he says.

Either way, Dildine adds that a little judicious personal experimentation isn't unreasonable. "Just because the evidence isn't good that something works, it doesn't absolutely mean it won't work for you. If there's no harm, why should I, as a doctor, be a jerk about telling you not to try it?"

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You're on the Verge of Getting a Cold: Can You Stop It in Its Tracks?

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F or those who want to give vitamin C or zinc (or both) a shot, it's easy enough to find supplements of either. One popular zinc supplement, for example, is Zicam, which provides quick-dissolving tablets specifically marketed as shortening cold duration.

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The common cold is a top reason for missed work and school days. Most of us have two or three colds per year, each lasting at least a week.

There's no real cure, but studies from the last several years show that some supplement containing zinc can help shorten the duratio n of cold symptoms by up to 40% - depending on the amount of the mineral in each dose and what it's combined with.

Zinc has an interesting back story. It wasn't even acknowledged as an essential mineral for human health until the 1970s. But that changed thanks to the work of Dr. Ananda Prasad - a 91-year-old doctor who, decades ago, had a hunch that led to a better understanding of zinc's role in immunity.

Back in the 1960s Prasad was studying a group of young men in Egypt who had not grown to normal heights and remained underdeveloped in other ways, too. Prasad wondered if the problem might be a lack of zinc.

When Prasad gave them zinc supplements, the men grew significantly taller.

"I couldn't believe it," he says. Prasad had never expected such significant growth.

Some scientists challenged his findings, at the time, questioning the idea that zinc deficiency could even occur in humans. "It was controversial," Prasad says.

But he pressed on with his research and began to document the ways zinc influences immunity.

Eventually, in the 1970's, the National Academy of Sciences declared zinc an essential mineral, fundamental to many aspects of cell metabolism. NAS established a recommended daily allowance, which is the daily amount that's sufficient for good nutrition. ( Most of us today get plenty of zinc from foods such as beans, nuts, whole grains and fortified cereals.)

Prasad says he felt vindicated by this action. "Absolutely," Prasad told us from his home in Michigan, where he's a researcher and professor at Wayne State University School of Medicine in Detroit.

What came next in his career may be just as surprising. Prasad had demonstrated that zinc had an effect on immunity - so he figured that it might help against a ubiquitous scourge - the common cold.

To test this theory, he collaborated with a scientist at the University of Michigan - who was skeptical.

"When I first heard this - I actually told his research assistant: I think he's losing it," recalls James T. Fitzgerald, who has expertise in designing research studies.

Fitzgerald agreed to help with a double-blind, placebo-controlled trial. They identified participants in the Detroit area who had developed colds and gave them zinc lozenges. Neither the participants nor the researchers knew who was given the real thing, and who was given the placebo.

"Lo and behold, when I did the analysis it indeed did shorten common cold symptoms by about two or three days, " Fitzgerald says. "I was stunned by that result."

Zinc's been studied for years, and not all studies have pointed to a benefit, but that may be because of the dosage and composition of the lozenge used.

Most recently, a meta-analysis published in 2017 by Harri Hemilä at the University of Helsinki concludes that 80 to 92 milligrams per day of zinc, given at the onset of cold symptoms, reduced duration of the common cold by 33%.

The study finds that two different zinc compounds - zinc acetate and zinc gluconate - are botheffective. And there's no evidence, the researchers say, that increasing those doses of zinc (to 100 milligrams per day or more) leads to any greater efficacy.

Great news. Nonetheless, "it's quite difficult to instruct patients," says Hemilä, the author of the meta-analysis. That's because zinc cold formulations sold at drugstores often contain multiple ingredients that can undercut the zinc's effectiveness. For instance, the lozenges "should not contain citric acid," Hemilä says, because it binds with the zinc in a way that keeps the mineral from being released. In addition, he says, the mechanism of action is not well understood, so scientists can't say exactly how it works.

The lozenge used in the Michigan study is not available commercially, Fitzgerald notes. "It was especially designed for the study."

So, do check each package's list of ingredients and the size of the zinc dose, the scientists recommend. Because not all products are going to help.

And remember, other common sense measures can help ease your cold symptoms too - like getting good sleep and drinking extra fluids -- or keep it from deteriorating into something worse.

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Taking Zinc Can Shorten Your Cold. Thank A 91-Year-Old Scientist For The Discovery

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The common cold is a top reason for missed work and school days. Most of us have two or three colds per year, each lasting at least a week.