Colds and the flu

An in-depth report on the diagnosis, treatment, and prevention of colds and flu.

Highlights

Colds and influenza (flu) are respiratory tract infections caused by airborne viruses:

• The common cold is caused by more than 200 viruses. Rhinovirus is the most common cause.
• Influenza in humans is caused mainly by two viruses: influenza A and influenza B. There are many different strains and flu viruses constantly mutate.

Colds and the flu can share similar symptoms such as stuffed up or runny nose but there are also differences:

• Flu symptoms are usually more severe than the cold
• Fever is common with the flu but rare with colds
• Headache and extreme exhaustion are more common in flu than colds
• Sore throat is more common with colds than flu

The viruses that cause colds and flu are spread by direct contact with secretions or by being close to an infected person who coughs or sneezes. You can reduce your risk of catching these germs by:

• Frequently wash your hands and use hand sanitizer.
• Avoid touching your hands to your eyes, mouth, or nose.
• Clean and disinfect commonly touched surfaces and objects.
• Avoid crowded environments during cold and flu season.
• Get a flu vaccine.

The best way to avoid getting influenza is to get an annual flu vaccine. The U.S. Centers for Disease Control and Prevention (CDC) recommends that everyone over the age of 6 months receive a flu vaccine every year. There are two types of flu vaccines:

• Injectable flu vaccines (flu shots) use inactivated (killed) viruses. Flu shots may either protect against 3 strains of influenza (trivalent) or 4 strains (quadrivalent). Some high-dose flu shots are recommended specifically for people over age 65.
• Nasal spray flu vaccines use live attenuated (weakened) influenza viruses. They protect against 4 strains of flu viruses. In 2014, the CDC recommended the nasal spray vaccine for healthy children ages 2 to 8 years old.

Colds and flu are usually treated by resting and drinking lots of fluids. Most cases of flu and colds resolve by themselves within a few weeks. However, some people are at increased risk for developing serious influenza complications. They include young children, older adults, pregnant women, and people with chronic health conditions.

In 2015, the CDC recommended antiviral drug treatment for people at high risk for flu complications:

• Recommended antiviral drugs are oseltamivir (Tamiflu) and zanamivir (Relenza). Peramivir (Rapivab) is a new antiviral that is given intravenously to hospitalized patients.
• Antiviral drugs can reduce symptom severity and shorten the duration of illness by 1 to 2 days. They work best when started within 2 days of symptom onset. These drugs have a number of side effects.
• Antibiotics are not used to treat viral infections like the flu, but may be used if secondary bacterial infections develop.

Introduction

The common cold and flu (influenza) are respiratory tract infections that are caused by airborne viruses. Colds and flu primarily affect the upper respiratory tract (nose, sinuses, and throat). The flu can also affect the lower respiratory tract (airways and lungs).

The viruses that cause colds and flu are generally spread by:

• Direct contact with secretions (such as hand-to-mouth)
• Inhaling the virus after an infected person coughs or sneezes

The Common Cold

The common cold (medically known as rhinopharyngitis or nasopharyngitis) is the most common upper respiratory tract infection. More than 200 different viruses can cause colds. The most common cause are rhinoviruses. Less common viral causes include coronaviruses, influenza, and parainfluenzas.

Cold symptoms develop 1 to 3 days after being exposed to the virus and usually resolve within 10 days. Cold symptoms are nearly always less severe than those of the flu.

Influenza ("The Flu")

The flu is caused by the influenza virus. There are two main types:

• Influenza A is the most widespread flu virus. It can infect birds, pigs, and humans (and a few other mammals). Influenza A is the major cause of global outbreaks of flu (pandemics).
• Influenza B virus infects only humans. It is less common and generally causes less severe illness than type A.

Influenza A is further categorized by subtypes based on two proteins that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N). There are 18 "H" types and 11 "N" types. Influenza A viruses are also categorized by their hosts (humans, birds, and swine).

Two particularly important and serious human influenza A viruses are H1N1 and H3N2. The annual seasonal flu vaccine always includes an influenza A H1N1 strain, an influenza A H3N2 strain, and one or two influenza B strains. (Influenza B viruses are not subtyped.) Unfortunately, influenza A frequently mutates and in some years the vaccine is not as effective.

Most of the time, avian flu stays contained within bird populations. However, some of these strains can be transmitted directly from birds to people and cause serious or even fatal illness. Avian influenza H5N1 and H7N9 are two bird flu virus that have infected humans. Both viruses can cause serious, and fatal, disease due to pneumonia and respiratory distress.

Most cases of transmission involved people who came into direct contact with infected birds. In their current form, avian influenza viruses do not easily spread from person to person. However, if the viruses continue to mutate, more cases of human transmission of avian flu may occur, which would increase the possibility of a global outbreak (pandemic).

The 2009 H1N1 pandemic developed from a new virus that emerged from a combination of bird, pig, and human strains. It spread as the virus passed from pigs to humans, and then from people to people. H1N1 is now established as a human seasonal flu virus that also circulates in pigs.

Risk Factors

Seasons

Colds and the flu tend to occur more often in the winter. Flu season typically starts in October and lasts into mid-March.

Colds and flu are more likely to be transmitted in winter because people spend more time indoors and are exposed to higher concentrations of airborne viruses. Dry winter weather also dries up nasal passages, making them more susceptible to viruses.

You cannot get colds or flu by getting cold, wearing insufficiently warm clothes in warm weather, or going outside with wet hair.

Everyone is at risk for colds and the flu, but certain people have a greater risk for experiencing complications.

Age

Young children (under age 4) and older adults (over age 65) have a higher risk for colds and flu and their associated complications.

Medical Conditions

People who have certain medical conditions or treatments are at increased risk for complications from cold and flu. They include:

• AIDS, cancer and other conditions that weaken the immune system
• Asthma, chronic lung disease, or other respiratory conditions
• Seizure disorders (epilepsy) and other neurological disorders (stroke)
• Heart disease, diabetes, and other chronic conditions
• Pregnancy, especially in 2nd and 3rd trimesters
• Medications, such as long-term steroid use

Complications

Colds and flu usually cause mild illness with symptoms resolving within 7 to 10 days. In some cases, especially with people who have existing health issues, these viruses can cause more serious health problems.

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Symptoms

Symptoms of colds and flu usually emerge 1 to 3 days after exposure to the virus:

• A cold nearly always begins with throat irritation and nasal congestion and sneezing. Most symptoms resolve within 10 days, but cough can last up to 2 weeks.
• The flu usually begins rapidly with chills, fever, muscle aches, fatigue, and fever in addition to symptoms similar to the common cold. Children may experience vomiting and diarrhea. Most symptoms subside after a few days. Cough and fatigue may linger for several weeks.

Differentiating between a cold and flu can be difficult. Cold symptoms are nearly always less severe than those of the flu.

Diagnosis

Colds and flu are generally diagnosed based on clinical symptoms. The presence of fever can help distinguish cold from flu. Your health care provider will also make note if your symptoms coincide during a time of active influenza outbreaks in your area.

In certain cases, your provider may want to do a laboratory exam for influenza by using a nasopharyngeal culture. This involves using a swab to take secretion samples from your nose or throat. This test should be done within 4 days of your first symptoms.

Some tests using nasopharyngeal samples can produce results within 15 minutes. However, these rapid tests often produce false negative results (indicating no influenza when influenza is present) when flu activity is high, and false positive results (indicating influenza when no influenza is present) when flu activity is low. In addition, most of these tests do not distinguish between influenza A or B, or provide information on influenza A subtypes.

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Some differences between colds and allergic rhinitis symptoms are:

• Allergic rhinitis symptoms begin right after exposure to an allergen (such as pollen, grass, or dust mites). Colds or flu develop several days after exposure to a virus
• Allergic rhinitis symptoms last as long as you are exposed to the allergen. Cold and flu symptoms generally last 2 to 14 days
• The nasal discharge in allergic rhinitis is usually clear. In colds or flu it is yellow
• Colds and flu are often accompanied by aches and pain. A fever is a definite sign that the condition is a cold or flu and not allergic rhinitis.

Prevention

The viruses that cause colds and flu are transmitted through the air. You can catch a cold or flu when you breathe in tiny airborne droplets from the coughs or sneezes of someone who is sick. You can also catch a cold or flu if you touch something with the virus on it and then touch your mouth, nose, or eyes.

Symptoms of colds and flu start about 1 to 4 days after the virus enters your body. You can be contagious to others a day before symptoms begin and even for a few days after they begin to fade. Most symptoms typically last about a week. You are most contagious when your symptoms are at their worst.

Preventing the Spread of Colds and Flu

You can reduce your chances of getting sick (and prevent infecting others if you are sick) by:

• Frequently wash your hands with soap and water.
• Use hand sanitizer
• Use paper towels instead of sharing cloth towels.
• Avoid touching your hands to your eyes, mouth, or nose.
• Disinfect your environment by cleaning commonly touched surfaces and objects.
• Avoid crowded environments during cold and flu season.
• Get an annual flu vaccine.

If you are sick with a cold or flu, you can reduce your chance of infecting others by:

• Staying home when you are sick and for at least 24 hours after your fever is gone.
• Coughing or sneezing into a tissue or into the crease of your elbow (not the air)
• Avoiding sharing food, utensils, cups, or plates
• Wearing a face mask when you are around people

Influenza Vaccines

The best way to avoid getting influenza is to get a seasonal flu vaccine. (There is no vaccine for the common cold.) The U.S. Centers for Disease Control (CDC) recommends annual flu vaccination for everyone age 6 months and older.

The flu vaccine is available as an injection (flu shot) or as a nasal spray. There are many types of these vaccines. Some vaccines are only appropriate for certain people based on age and other factors. If you have a severe allergy to eggs, or have certain chronic health conditions, talk to your health care provider before getting vaccinated. There are flu shots available that do not contain egg substances.

The flu vaccine is reformulated every year based on a prediction of which strains of flu are most likely to occur. Flu viruses constantly mutate and change in a process called antigenic drift. (Antigens are foreign molecules that the immune system specifically recognizes as alien and targets for attack.) Unfortunately, sometimes flu strains can change within the course of a flu season, so in some years the vaccine is not as effective.

Even a mismatched vaccine can still provide protection against related viruses. A flu vaccine contains 3 or 4 flu strains, so you will always be getting some protection. Even if you catch the flu, a vaccine can help by giving you a milder case of flu than you would have had if you were not vaccinated.

You should try to get a flu vaccine as soon as it becomes available for the season (usually in September or October), but even if you get it later in the season it can still help. It takes about 2 weeks after you get vaccinated for the vaccine to take full effect.

The nasal spray vaccine is called a live, attenuated influenza vaccine (LAIV). It contains live, but weakened, flu viruses. Flu shots contain inactivated (not live) viruses.

Flu shots contain and protect against either 3 (trivalent) or 4 (quadrivalent) viruses. The LAIV nasal spray contains 4 viruses. Flu shots also come in different dose strengths. A high-dose flu shot is available specifically for people age 65 and older.

A trivalent flu vaccine protects against 2 influenza A viruses (H1N1 and H3N2) and one influenza B virus. A quadrivalent flu vaccine protects against the 2 influenza A viruses plus 2 influenza B viruses. The addition of a second influenza B virus provides broader protection.

Treatment

Most cases of colds and flu can be treated at home by resting and drinking fluids. Home remedies and some medications cannot help cure cold or flu, but may help provide symptom relief.

You should contact your health care provider if:

• Your symptoms get worse or do not improve after 7 to 10 days
• You are at risk for having complications

People who are at risk for complications include those with chronic health conditions (asthma, diabetes, heart disease, and obesity), people over age 65, children younger than age 5, and pregnant women.

If a high-risk person has the flu, their health care provider may recommend an antiviral medication. Antibiotics are not prescribed for treating viral infections such as colds and flu. They may be prescribed if a secondary bacterial infection develops.

Home Remedies

There are many simple home remedies that can help you feel better. They include:

• Rest as much as you can to help your body fight off the infection.
• Drink plenty of fluids, especially water.
• Consume hot fluids such as tea, broths, and soup. There is evidence that chicken soup can indeed help with mucous congestion and other cold and flu symptoms. Ginger or peppermint teas can be very soothing.
• Add honey to tea or hot lemon water. The American Academy of Pediatrics recommends honey to treat coughs in children.
• Eat spicy foods with garlic or chili peppers to help clear nasal congestion.
• Gargle with salt or lemon water to treat sore throat. Throat sprays or lozenges can also help.
• Try steam inhalation for cough and sinuses. Place a towel over your head while leaning over a bowl or pot of hot (not boiling) water. Breathe deeply. You can add herbs or essential oils such as thyme, rosemary, eucalyptus, peppermint, or bay leaves.
• Use a vaporizer or humidifier.
• Do not smoke and avoid secondhand smoke.

A saline solution can be purchased in a spray bottle at a drug store or made at home. (Mix 1 teaspoon of table, Kosher, or sea salt with 2 cups of warm water. Some people add a pinch of baking soda.) If you prepare your own saline solution, use bottled or boiled water, not plain tap water. Perform the nasal wash several times a day. It can help moisten nasal passages and remove mucus.

A simple method for administering a homemade nasal wash is:

• Lean over the sink head down.
• Pour some solution into the palm of the hand and inhale it through the nose, one nostril at a time.
• Spit out the remaining solution.
• Gently blow the nose.

Neti pots have also become popular in recent years. Nasal irrigation with a saline solution through a Neti pot involves:

• Lean over the sink with your head tilted to one side.
• Insert the spout of the Neti pot in the upper nostril.
• Slowly pour the salt water into your nose while continuing to breathe through your mouth.
• The water will flow through the upper nostril and out through the lower nostril.
• When the water finishes dripping out, blow your nose.
• Reverse the tilt of your head and repeat the process with the other nostril.

Antiviral Drugs

Antiviral medications are given to treat influenza (not the common cold). These drugs are available only by prescription. They should be started within 2 days of symptom onset, although they can still be helpful if started later. These drugs can reduce symptom severity and help shorten the duration of illness by 1 to 2 days.

Anyone with influenza is a candidate for an antiviral drug, but these medications are most important for people who are severely ill or at risk for influenza complications. This includes young children, pregnant women, and people with compromised immune systems or chronic health conditions. People at risk for complications should receive immediate treatment with an antiviral drug without waiting for diagnostic testing to confirm influenza.

The Centers for Disease Control (CDC) recommends these antiviral drugs for treatment of influenza:

• Oseltamivir (Tamiflu) is available for adults and children as a pill or liquid that is taken for 5 days
• Zanamivir (Relenza) is available for adults and children as a powder that is inhaled that is taken for 5 days
• Peramivir (Rapivab) is administered intravenously in a single dose to hospitalized adults

These drugs are classified as neuraminidase inhibitors. Older antiviral drugs called adamantanes, which included amantadine and rimantadine, are no longer recommended because they are not effective for influenza B and have become less effective for influenza A.

Side effects may include nausea, vomiting, diarrhea, dizziness, headache, and behavioral changes. Zanamivir should not be used by people with breathing problems like asthma or COPD.

OTC Cough and Cold Medicines

Over-the-counter (OTC) medications are available without a prescription. Combination OTC cough and cold medicines contain more than one medicine, such as decongestants and antihistamines or pain relievers. (Antihistamines are often included in nighttime cold products to produce sleepiness.). Read the labels carefully to make sure you do not take too much of any one medicine. Ask your health care provider which cold medications are safe for you.

The FDA advises that cough and cold medicines should never be given to children younger than age 4. These drugs can cause serious side effects in young children.

• Oral decongestants contain pseudoephedrine or phenylephrine. Products that contain pseudoephedrine are kept behind the counter to prevent potential use for the illegal manufacturing of methamphetamine
• Nasal decongestants come in long-acting and short-acting forms. Long-acting brands often contain oxymetazoline and can last for up to 12 hours. Short-acting brands often contain phenylephrine and last for up to 4 hours. Ipatropium bromide can help relieve runny nose, but not congestion.

Nasal decongestants are applied directly into the nasal passages. Nasal forms work faster than oral decongestants and have fewer side effects. However, nasal decongestants should never be used for more than 3 days in a row due to risks for rebound congestion.

Oral decongestants provide temporary relief for up to 4 hours. They can cause side effects such as nervousness, dizziness, and sleepiness. People who have high blood pressure should never use standard pseudoephedrine or phenylephrine decongestants. They should use a product such as Coricidin HBP, which contains the antihistamine chlorpheniramine.

If you have high blood pressure, heart disease, diabetes, thyroid disease, prostate problems, or glaucoma talk to your health care provider before taking any oral or nasal decongestant. You may be at risk for side effects and complications.

• Cough suppressants (antitiussives) block the cough reflex. They often contain dextromethorphan.
• Cough expectorants loosen and thin mucus so you can cough it up. Guaifenesin is the ingredient used in expectorants.

Some cough products also contain decongestants and antihistamines. Antihistamines make you sleepy. Decongestants may actually worsen cough by thickening mucus.

Studies have questioned the effectiveness of OTC cough remedies. Medications that contain both a cough suppressant and an expectorant are not useful and should be avoided. As noted, never give cough or cold medicine to a child under 4 years without a pediatrician's approval.

If your cough lasts for more than 3 weeks, or is accompanied by shortness of breath, contact your health care provider. A lingering cough may be a sign of bronchitis or pneumonia.

Pain Relievers

Many people take medications to reduce the mild pain and fever associated with colds and flu. The most common types of over-the-counter pain medicines are acetaminophen (Tylenol, generic) and nonsteroidal anti-inflammatory drugs (NSAIDs). NSAIDs include ibuprofen (Advil, Motrin, generic), naproxen (Aleve, generic), and aspirin.

Important precautions for pain relievers include:

• Aspirin and aspirin-containing products should never be given to children or adolescents who have the flu or chicken pox. Reye syndrome is a serious and potentially fatal brain condition that can occur when children with viral infections are given aspirin. Never give aspirin to your child unless directed to do so by a health care provider.
• NSAIDs are generally safe when taken for a short period of time (less than 10 days). However, if you have heart conditions, high blood pressure, or history of stomach bleeding, talk to your health care provider.
• Acetaminophen in high dosages can cause serious liver injury. When taking combination medicines, always check the ingredients for the presence of acetaminophen, and make sure never to take more than the recommended daily dose of 3 grams (3,000 mg) in a single day.
• Always check product labels for recommended dosages for adults and children.

Vitamins, Minerals, and Herbal Supplements

For many years, vitamin C has been a popular remedy for the common cold. Research suggests:

• For most people, vitamin C supplements or vitamin C-rich foods do not reduce the risk of getting the common cold.
• However, people who take vitamin C supplements regularly might have slightly shorter colds or somewhat milder symptoms.
• Taking a vitamin C supplement after a cold starts does not appear to be helpful.

The recommended daily allowance of vitamin C for adults is 90 mg/day for men and 75 mg/day for women. High doses (over 1,000 mg/day) may increase the risk for kidney stones. Doses over 2,000 mg/day may cause nausea and diarrhea. Large amounts of vitamin C can interact with many medications including aspirin, acetaminophen, and blood-thinner drugs like warfarin.

Zinc lozenges are often marketed as cold remedies but it is unclear how effective they are. A review of 16 studies suggested that when started within 24 hours of symptom onset, oral zinc may shorten the duration of colds. However zinc lozenges do not appear to be helpful for preventing colds or reducing symptom severity.

Oral zinc has a number of side effects including bad taste and nausea. High doses of oral zinc (more than 40 mg/day) can cause vomiting, diarrhea, and serious side effects. Zinc can interact with a number of medications, including antibiotics. Zinc nose spray is not recommended because it can cause a loss of smell.

Resources

• www.cdc.gov/flu -- U.S. Centers for Disease Control and Prevention
• www.niaid.nih.gov -- National Institute for Allergy and Infectious Diseases
• www.who.int/csr/disease/influenza/en -- World Health Organization
• www.cdc.gov/vaccines -- Advisory Committee on Immunization Practices (ACIP)