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Influenza - FAQs:
How many times can a person become infected with the flu virus?
You are unlikely to get infected with the same exact strain of influenza (flu) more than once, but it is possible for a person to be infected with influenza (flu) virus more than 1 time in a season. This is because several different strains of flu virus circulate (go around) each year. Exposure to a particular strain of flu virus may help protect you against that strain in the future. But, it will not protect you from infection with other flu virus strains.
It's important to remember that there are many different types of flu viruses that are constantly changing. This means that antibodies (infection fighting cells) made against 1 strain of flu virus:
- Will become less effective (weaker) against new strains as they evolve (change) over time; and
- Will not protect you against different strains of the virus.
Several different types of flu viruses can go around each flu season, and the viruses change from flu season to flu season.
In addition to flu viruses, several other respiratory viruses also can circulate during the flu season and can cause symptoms and illness similar to those seen with flu infection. These non-flu viruses include rhinovirus (one cause of the "common cold") and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children, as well as a leading cause of death from respiratory illness in those aged 65 years and older.
What occurs when a virus changes and the result makes the antiviral drug less effective in treating it?
Influenza viruses are dynamic and are continuously evolving. Influenza viruses can change in two different ways:
- antigenic drift, and
- antigenic shift.
Influenza viruses are changing by antigenic drift all the time. Antigenic shift happens only occasionally. Influenza type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.
Antigenic drift refers to small, gradual changes that occur through point mutations in the two genes that contain the genetic material to produce the main surface proteins, hemagglutinin, and neuraminidase. These point mutations occur unpredictably and result in minor changes to these surface proteins. Antigenic drift produces new virus strains that may not be recognized by antibodies to earlier influenza strains. This process works as follows: a person infected with a particular influenza virus strain develops antibody against that strain. As newer virus strains appear, the antibodies against the older strains might not recognize the "newer" virus, and infection with a new strain can occur. This is one of the main reasons why people can become infected with influenza viruses more than one time and why global surveillance is critical in order to monitor the evolution of human influenza virus stains for selection of which strains should be included in the annual production of influenza vaccine. In most years, one or two of the three virus strains in the influenza vaccine are updated to keep up with the changes in the circulating influenza viruses. For this reason, people who want to be immunized against influenza need to be vaccinated every year.
Antigenic shift refers to an abrupt, major change to produce a novel influenza A virus subtype in humans that was not currently circulating among people (see more information below under Influenza Type A and Its Subtypes). Antigenic shift can occur either through direct animal (poultry)-to-human transmission or through mixing of human influenza A and animal influenza A virus genes to create a new human influenza A subtype virus through a process called genetic reassortment. Antigenic shift results in a new human influenza A subtype. A global influenza pandemic (worldwide spread) may occur if three conditions are met:
- A new subtype of influenza A virus is introduced into the human population.
- The virus causes serious illness in humans.
- The virus can spread easily from person to person in a sustained manner.
To learn more, see:
What is an over the counter medicine that one can take who may have the flu?
If you have flu-like symptoms, you can purchase the following medications, which are available at your local drug store or pharmacy. These generic drugs are commonly identified by their brand or over-the-counter name:
- Acetaminophen (Tylenol);
- Ibuprofen (Advil, Motrin or Nuprin); and
- Naproxen (Aleve).
If you are sick, see your doctor. Flu can lead to, or occur with, bacterial infections. Therefore, some people will also need to take antibiotics (drugs that kill bacteria) if they have:
- More severe or prolonged illness; or
- An illness that seems to get better, but then gets worse again.
From a business standpoint if there is a confirmed case of the flu in the home is it still safe to allow that employee to come to work if he/she is not sick? What are the chances of it being transferred to family members?
An employee with an ill household member may go to work. But it is especially important that these employees monitor themselves for illness.
Employees with school-aged children may need to stay home to care for their children. Employers should review leave policies for the flexibility to allow employees to stay home if they need to care for their children or other household members. If flu conditions are more severe, CDC guidance for school-aged children is that they should stay home for 5 days from the time someone in their home became sick. However, this guidance does not apply to adults.
People infected with seasonal influenza (flu) shed virus and may be able to infect others from 1 day before getting sick to 5 to 7 days after. This can be longer in some people, especially:
- Children;
- People with weakened immune (body defense) systems; and
- People infected with the 2009 H1N1 virus.
Many people with flu illness will continue shedding the flu virus 24 hours after their fevers go away, but at lower levels than during their fever. In some cases, shedding of the flu virus can be detected for 10 days or more by reverse-transcription polymerase chain reaction (RT-PCR). So, when people who have had flu-like illness return to work, school, or other community settings, they should:
- Continue to practice good respiratory etiquette and hand hygiene; and
- Avoid close contact with people they know to be at increased risk of flu-related complications.
Because some people may shed flu virus before they feel ill, and because some people with flu will not have a fever, it's important that all people:
- Cover their cough; and
- Wash their hands often.
What are the types of human flu viruses?
There are three types of human influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus caused a flu pandemic.
Influenza B viruses are not divided into subtypes. But, influenza B viruses can be further broken down into different strains.
Regular influenza A (H1N1), A (H3N2), and influenza B viruses are included in each year's seasonal influenza vaccine. Getting a flu vaccine can protect against flu viruses that are the same or related to the viruses in the vaccine. The seasonal flu vaccine does not protect against influenza C viruses.
How common is it for humans to be infected with flu viruses that normally spread in swine?
In the past, CDC received reports of approximately one human infected with influenza viruses that normally spread in swine every one to two years, but more recently, these cases have been detected more frequently. From December 2005 through December 2011, 35 cases of human infection with variant influenza have been reported.
The increased detection and reporting of these cases could be occurring for a number of reasons, including one or more of the following factors:
- First, pandemic preparedness efforts have improved state level surveillance and laboratory capacity to detect novel viruses in the United States.
- Second, in 2007, novel flu virus infections were made domestically and internationally reportable.
- And three, it’s also possible that there is a true increase in the number of these cases, possibly occurring from exposure to infected swine or through subsequent, limited human-to-human transmission.
Why are human infections with variant viruses of concern?
Influenza viruses that infect pigs are different from human influenza viruses. Thus, influenza vaccines made against human influenza viruses are generally not expected to protect people from influenza viruses that normally circulate in pigs.
In addition, because pigs are susceptible to avian (bird), human, and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus that could spread easily from person-to-person.
This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A virus to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person-to-person, an influenza pandemic can occur. This is what happened in 2009 when an influenza A H1N1 virus with swine, avian and human genes emerged in the spring of 2009 and caused the first pandemic in more than 40 years.
What symptoms do people have when they are infected with variant viruses?
People who have been infected with variant viruses have had symptoms similar to the symptoms of regular human seasonal influenza. These include:
- fever,
- lethargy,
- lack of appetite and
- coughing.
Some people also have reported runny nose, sore throat, eye irritation, nausea, vomiting, and diarrhea.
What medications are available to treat variant flu infections in humans?
There are four different antiviral drugs that are licensed for use in the United States for the treatment of influenza:
- amantadine,
- rimantadine,
- oseltamivir and
- zanamivir.
In the past, most variant influenza viruses had been susceptible to all four drugs, however the most recent variant influenza viruses isolated from humans are resistant to amantadine and rimantadine. At this time, CDC recommends the use of oseltamivir or zanamivir for the treatment and/or prevention of infection with these variant influenza viruses.
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