Women in bed with flu

[Updated 1/24/18]. The 2017-2018 flu season is shaping up to be big and bad. According to the Center for Disease Control’s December 2017 FluView Activity Update, 46 states have reported and additional 4 regional activity. There have been almost 4,000 hospitalizations due to laboratory-confirmed influenza. Most have been among people 65 and older, those who are 50-64, and children younger than 5. But serious illness and hospitalizations can occur at any age. The current death count is not yet available, but some areas like San Diego, California have reported a significant number, again mainly elderly people many of whom were in nursing homes.

To help you get through this year’s flu season, we’ve pulled together the answers to the most commonly asked questions about influenza, its prevention, and its treatment.

What is the flu anyway?

Flu is an infection that is caused by viruses in the influenza family. There are two main types of influenza virus that cause epidemic disease in humans, A and B. A notable characteristic of these viruses, particularly type A, is the changeable nature of glycoproteins that reside on the surface of the virus. The two most important glycoproteins are hemagglutinin (H) and neuraminidase (N), and there is a reason why you should know about them.

The H and N glycoproteins serve as antigens to which the body forms antibodies to help fight off the infection. A number of different viral strains circulate each flu season. The names of the A viruses identify the variants of their glycoproteins. You may hear, for example, that A/H1N1 virus or an A/H3N2 virus are dominant in a given year as they are expected to be this flu season.

Public health organizations, like the Centers for Disease Control, do surveillance to determine which viruses are likely to be in circulation each flu season so that vaccine manufacturers can create a vaccine that targets those variants. This is why we have to get flu vaccines every year instead of once in a lifetime.

Which flu viruses are causing the flu this season?

The CDC reports that most (85%) of the influenza-positive samples submitted tested during the week ending December 30, 2017, showed infection with influenza A viruses; only 15% were due to B viruses. The most frequently identified subtype of virus was the H3N2 type of influenza A. This is important for several reasons. This subtype is associated with more severe illness and more hospitalizations and deaths. In addition, because it hasn’t been predominant in circulation since 2009, children born after that time will not have pre-formed antibodies to it so they may get sicker than in prior years when H3N2 was not circulating.

Most of the viruses collected from the US between the start of flu season (Oct 1) and the end of the year were “antigenically and genetically” similar to the reference viruses for the 2017-18 Northern Hemisphere vaccine. We do not yet know the effectiveness of the vaccine used this year, but there is some concern because the vaccine used in Australia earlier this year has been reported to have been significantly less effective that in “normal” flu years. There is some suggestion that this was related to the production of the vaccine and not to a mismatch between the virus and the vaccine antibodies.

When is flu season?

Although sporadic cases of flu are reported year round, in North America, the season is usually considered to start in October and run through March, with the peaks in December and February. Some cases can occur as late as May.

CDC graphi peak flu activity
Graphic from cdc.gov/flu/about/season/flu-season.htm

The exact timing is different depending on where in the country you live. There are online flu trackers that you can check to see if the flu is present in your part of the country or the world. The CDC’s Weekly Influenza Map tracks the flu by state and territory and is updated weekly based on reports from epidemiologists. The World Health Organization’s (WHO) FluNet tracks circulating flu and reports the results by country. Flu Near You publishes a map based on reports of flu-like symptoms from people all over the country. It provides a helpful overview even though the flu cases have not been verified via laboratory tests. Anyone can sign up to be a flu symptom reporter. It only takes a few minutes every week.

Should I get a flu shot?

The best way to avoid getting the flu is to get an annual flu shot. The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months and older, including pregnant women, should receive the flu shot every year unless they have a specific contraindication to any component of the influenza vaccine. Allergy to eggs is no longer considered a contraindication, although people with severe egg allergy should be vaccinated in a medical setting with a healthcare professional who can manage severe allergic reactions.1

The effectiveness of the vaccine depends on how well the vaccine matches the strains of influenza circulating in that year. When there is a good match, vaccine effectiveness ranges between 40-60%. It is important to understand that even when the match is poor, the vaccine has been shown to reduce the risk of hospitalization and death from influenza.

There are a variety of vaccines available including inactivated influenza virus vaccines that contain either three (Trivalent Vaccine) or four (Quadrivalent Vaccine) different flu antigens (a mix of type A and B strains). Recombinant vaccines are manufactured using recombinant DNA technology that does not require the use of the influenza virus or chicken eggs. The live attenuated vaccine (FluMist) is not recommended for use again this year due to concerns about its effectiveness against one of the strains expected to circulate this flu season.

Is there a special vaccine for seniors?

Because immune defenses decline with age, seniors should receive either Fluzone, a high-dose vaccine (four times the antigen compared with standard dose) or Fluad, a vaccine with an adjuvant, MF59, that enhances the immune response. Flublok is a recombinant vaccine that is produced without the use of the influenza virus or chicken eggs. It contains three times as much flu antigen as standard dose vaccines and has been shown to reduce the chance of getting the flu by about 30% when compared to the standard dose inactivated vaccine.

When should I get a flu shot?

Although you can be vaccinated at any time during the flu season or while the flu virus is circulating in your community, you should try to get vaccinated early because it takes about 2 weeks for vaccine-induced antibodies to reach their maximum level. Although antibody levels fall in the months after vaccination, revaccination during a single flu season is not recommended.

Are there any special considerations for children?

Children between the ages of 6 months and 8 years old, who are getting vaccinated for the first time or who have not had at last 2 prior doses of a tri- or quadrivalent vaccine before July 1, 2017, should receive 2 doses of any inactivated vaccine at least 4 weeks apart. Otherwise, only one dose is required.

Flu shots during pregnancy?

The flu can be more severe in pregnant women so they should definitely consider getting vaccinated. Although there was a concern during the 2010-2011 and 2011-2012 flu seasons that the flu vaccination could cause spontaneous abortion, a causal relationship has not been proven.1

How do I know if I have the flu?

People often confuse the flu with other viral diseases like colds and viral gastroenteritis. Symptoms of the flu characteristically come on suddenly and usually include a cough, fever, headache, sore throat, muscle aches (myalgias) and chills. A large retrospective study showed that when influenza was known to be going around in a community, the presence of both a cough and fever within 48 hours of developing symptoms was a strong indicator of flu (positive predictive value – 79%). The presence of sneezing makes the diagnosis of flu less likely.

Colds is an upper respiratory viral illness characterized by a runny nose, congestion, sneezing, sore throat, mild body aches, and sometimes mild headaches and a low-grade fever, colds don’t usually cause high fever or significant headaches and the flu hardly ever causes an upset stomach. The so-called stomach flu is not flu at all, rather it is a viral gastroenteritis unrelated to infection with an influenza virus.

Definitive diagnosis of the flu requires a laboratory test that may be performed quickly in your doctor’s office or sent to an outside laboratory. The latter tests are more sensitive (miss fewer flu infections) than the former but the results take longer to obtain (hours to days). In the midst of a documented flu outbreak, the illness may be diagnosed based on the presence of characteristic symptoms without obtaining a lab test.

How long does it last?

Symptoms of the flu usually last about 1 to 2 weeks with the first 3-4 days being the worst.

Is flu a serious infection?

Although the flu is an annoyance for most of us, it can be serious or even deadly in others. In fact, the flu is the most frequent cause of death from a vaccine-preventable disease in the United States. Somewhere between 200,000 to 710,000 people are hospitalized and 12,000 to 56,000 die each year because of the flu and its complications, particularly pneumonia. Infants, people over 65, and those with chronic illnesses are at particular risk. See the section on Treatment (below) to learn about high-risk conditions.

Why do some people die from flu

There are three major reasons why people die from flu:

  • They get a secondary bacterial infection, such as pneumonia. Streptococcal infections are common culprits, but other types of bacteria, such as staphylococcus, may also be involved. Sometimes these bacterial infections are complicated by the development of sepsis, an often fatal syndrome that involves widespread damage to many of the body’s organs.
  • The flu exacerbates underlying medical conditions, such as diabetes, asthma, or COPD. For example, an individual with diabetes be unable to stay hydrated and has a worsening of an already compromised renal function
  • They may have an overwhelming immune response that leads to a cytokine storm. Cytokines are hormone-like proteins that serve as modulators of the immune response. A potentially fatal situation occurs when a positive feedback loop between cytokines and white blood cells leads to very high levels of cytokines that damage the body tissues and organs.

What is a flu pandemic?

A major change in the character of the virus antigens occurs periodically. This is known as an antigenic shift. When it occurs, the population may not have any pre-formed antibodies from prior infections and there often isn’t a vaccine. Because of this, a large number of people may be infected as the virus moves through the population. This is known as a pandemic.

The most severe pandemic in modern times occurred in the 1918-19 flu season. It is often called the Spanish flu. More than 500 million people were infected worldwide and an estimated 20 to 50 million people died in that pandemic. Since then, there have been 5 other pandemics, the most recent in North America was in March 2009. Although some experts are worried that the 2017-18 flu season could be serious because of increased flu cases that have occurred in Australia this year, a pandemic is not anticipated at this point.

What is the treatment for flu?

The primary treatment of the flu is aimed at reducing symptoms, such as acetaminophen for fever, muscle aches, sore throat, and headaches; drinking fluids so you don’t get dehydrated, and taking it easy and getting plenty of sleep to combat weakness and exhaustion.

Two classes of antiviral drugs are available for treatment and prevention of influenza:

Although the CDC reports that the majority of viruses tested for resistance to the neuraminidase type of antiviral medications, do indeed show sensitivity, it is important to note that the impact of these drugs is relatively modest. If they are started within the first 24-48 hours of the onset of symptoms, they may reduce the duration of the flu by a half day to three days. Studies suggest that antiviral therapy also reduces the incidence and severity of complications and length of stay of people, such as older adults, who require hospitalization. Some question if there is any benefit of these drugs for healthy, immunocompetent individuals. The CDC states that “most people who are otherwise healthy and get the flu do not need to be treated with antiviral drugs.”

The Infectious Disease Society of America (IDSA) and the CDC recommend that individuals with evidence of pneumonia or severe disease that requires hospitalization be treated with an antiviral as promptly as possible. In addition, people with the following high-risk conditions should also be treated:

Following is a list of all the health and age factors that are known to increase a person’s risk of getting serious complications from the flu:

  • Asthma
  • Neurological and neurodevelopmental conditions
  • Blood disorders (such as sickle cell disease)
  • Chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis)
  • Endocrine disorders (such as diabetes mellitus)
  • Heart disease (such as congenital heart disease, congestive heart failure, and coronary artery disease)
  • Kidney disorders
  • Liver disorders
  • Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders)
  • People who are obese with a body mass index [BMI] of 40 or higher
  • People younger than 19 years of age on long-term aspirin therapy
  • People with a weakened immune system due to disease or medication (such as people with HIV or AIDS, or cancer, or those on chronic steroids)

Other people at high risk from the flu:

  • Adults 65 years and older
  • Children younger than 5 years old, but especially children younger than 2 years old
  • Pregnant women and women up to 2 weeks after the end of pregnancy
  • American Indians and Alaska Natives
  • People who live in nursing homes and other long-term care facilitiesIndividuals who are morbidly obese (BMI > 40).
  • Pregnant women and women up to two weeks postpartum.

How can I avoid spreading the flu?

Other things you should do if you get the flu is to stay at home when you are sick so that you don’t spread it to others. To avoid spreading the flu to family members or others if you have to go out, use a face mask and wash your hands frequently, particularly if you rub your nose, sneeze, or cough. Be sure to keep doorknobs and counters clean as the flu virus can persist on hard surfaces for up to 24 hours. They only survive about 15 minutes on tissues. You are usually contagious from the day before you start getting symptoms until about 5-7 days after you start feeling sick. This is variable, however, and there is some evidence that older people and immunocompromised people may be contagious for a longer period of time.

References without links:
1. Influenza Vaccine for 2017-2018. The Medical Letter on Drugs and Therapeutics, October 9, 2017. Vol 59:163-165 (requires a subscription)

This post was originally published on 10/18/17. It was reviewed and updated on 1/11/18.


  1. Thanks for sharing this information with us. Other things you should do if you get the flu is to stay at home when you are sick so that you don’t spread it to others.

  2. Influenza is a disease that is caused by infections in the flu family. There are two principal types of flu infection that reason epidemic sickness in people, A and B. An outstanding normal for these infections, especially type A, is the variable idea of glycoproteins that live on the surface of the infection. The two most vital glycoproteins are the hemagglutinin (H) and neuraminidase (N), and there is a motivation behind why you should think about them.


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