How the Influenza Vaccine Works


By Barry C. Fox, M.D.University of Wisconsin

After the 1957 Asian flu, the descendants of the 1918 strain of influenza had seemingly disappeared from human circulation until a mysterious event happened in 1977. The 1977 swine flu first emerged in New Jersey and killed several soldiers. This was the impetus for the urgent creation of an influenza vaccine that 40 million US citizens received in record time.

Ill woman sitting on a sofa covered with a blanket and blowing her nose in a napkin.
The symptoms of influenza-like illnesses can be confused as influenza itself. (Image: fizkes/Shutterstock)

Controversy around the Influenza Vaccine

This vaccination effort was shrouded in controversy because it may have caused a slight increase in the incidence of a very rare neurologic disorder, Guillain-Barré syndrome. Also, the pandemic never materialized. 

So, the side effects of the vaccination overshadowed the positive aspects of the rapid vaccine development and mass public health response. It made many people reluctant to get routine flu vaccinations for years to come after that event.

Learn more about the life cycle of a virus.

What Makes Influenza Stand Out from Other Respiratory Illnesses?

The term influenza-like illness, or ILI, is used to describe the symptom complex. There is usually an incubation period of 1 to 4 days between the time the virus is acquired and the time the illness begins. Not everyone with ILI has true influenza. Many viruses can mimic influenza, and several go by the strains of virus known as para-influenza 1, 2, and 3.

Usually, ILI begins with a runny nose and upper respiratory congestion for 24 hours. This is while the virus is gaining entry into the surface lining of the nose and mucous membranes of the head. In the next 24 hours, a sore throat and headache symptoms begin. 

The headache of true influenza seems to be relatively distinctive compared with other ILI, usually towards the front of the head and with the distinction that moving your eyes back and forth in the sockets elicits discomfort in your eyes. Fever usually begins during the second day. But this is just the beginning. Within the next 24 hours, the virus invades the bloodstream, breaking through the respiratory linings. 

True influenza virus has a propensity to invade muscles, sometimes leading to agonizing muscle aches and feelings of inability to move. The virus also descends into the upper airways of the lung, usually causing a dry, non-productive cough with white phlegm. However, primary influenza is unusual, but it can occur in very young children, elderly adults, or patients with compromised immune systems.

This is a transcript from the video series An Introduction to Infectious DiseasesWatch it now, on Wondrium.

Fighting the Virus

Two doctors diagnosing a male patient’s symptoms in a hospital setting. The doctors are wearing protective suits and masks. The patient is also wearing a mask.
Influenza leads to the hospitalization of more than 200,000 people yearly in the United States. (Image: Hananeko_Studio/Shutterstock)

To fight the virus, the body now starts to mount a higher fever—trying to kill the virus—usually of about 102 to 103 degrees. The resetting of the body’s thermostat can often lead to episodes of shaking chills that may feel uncontrollable.

Even three days into the illness, relief does not come quickly. The main symptoms persist for up to another 72 hours before finally scaling down on the seventh day of the illness. It may still take three to five days to get back to feeling “normal”.

While most of us will recover, presently, influenza leads to the hospitalization of more than 200,000 people yearly in the United States.

WHO-made Vaccines 

With all of these miserable symptoms, who wouldn’t want to prevent this illness with vaccination? While we are talking about vaccination, it’s worth mentioning that there has been an advance in the vaccine, and it has had more opportunities for successful protection.

Other changes to prior vaccination policy include allowing most patients with egg allergies to still be vaccinated, recommending that children ages 2 to 8 receive the alive, rather than killed, flu vaccine, and preferring a “high dose” vaccine for those over the age of 65, possibly for better prevention efficacy.

Flat lay of syringes, protective medical masks, and bottles for influenza vaccine.
Vaccines for influenza are based on data gathered from over 100 centers across the globe. (Image: Regina Burganova/Shutterstock)

In general, the northern hemisphere takes its cue from the vaccine composition from the southern hemisphere and vice versa since the seasons are reversed. In February, they decided on the vaccine for the northern hemisphere for the next season based on information gathered from influenza centers in 100 countries and other partners who conduct global surveillance year-round.

You might be wondering, since the manufacturing of the vaccine takes several months, how the World Health Organization determines which strains of influenza will be targeted in the vaccine for the fall? This is an important decision and involves influenza experts from all over the world. If the wrong strain is chosen for production, the efficacy of the vaccine may be more limited.

Learn more about malaria and tuberculosis, the global killers.

The Future 

Researchers in Madison have carefully studied genetic databases of avian flu that most closely contained proteins that resemble the 1918 Spanish flu. They use an animal model—ferrets—that can contract and transmit influenza and are a close model of human transmission.

The researchers intentionally altered the virus’s genetic code to within 3 percent relatedness of the 1918 Spanish flu. This enabled the virus to have receptors that would allow direct respiratory droplet transmission of influenza from ferret to ferret. These mutations involved only 7 tweaks of the virus.

This creation of an artificially virulent strain of avian flu generated tremendous international scrutiny. The Wisconsin researchers claimed that we needed to know this information to advance our knowledge of how pandemics begin, with the hope of designing better vaccines. 

They did test the antiviral oseltamivir against the mutant strain, and they, fortunately, found the drug was effective. This supports the pandemic preparedness belief that stockpiling anti-influenza drugs is important.

Common Questions about How the Influenza Vaccine Works

Q: What made the Influenza vaccine controversial in the US?

Because public health care authorities responded fast, there was nothing to show for what could have happened if the influenza vaccine didn’t exist. This led to rare side effects of the vaccine overshadowing the benefits.

Q: How does WHO decide which strain of influenza to concentrate on for the next vaccine?

Since the seasons are reversed in the two hemispheres, each is decided based on the influenza vaccine composition in the other hemisphere. Around 100 influenza centers across the globe gather data for this purpose.

Q: How does the body fight the influenza virus?

To fight the influenza virus, the body starts to mount a higher fever—trying to kill the virus. The main symptoms persist before finally scaling down on the seventh day of the illness. It may still take three to five days to get back to feeling “normal”.

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