Influenza: Facts, myths and prevention

Mar 24, 2018

As we set out to examine influenza on "Take Care," we wanted to start out with the basics. Flu is one of those illnesses that carries with it a lot of baggage in the form of myths and tall tales. After all, hasn't anyone ever told you that you can get the flu from the vaccine? (You can't.)

Dr. Angela Campbell, medical officer in the influenza division of the CDC, joined us to share her insight into this year's flu virus and how to prevent contracting it.

Campbell said the onset of flu can be sudden, with possible symptoms including fever, cough, sore throat, body aches, headache and fatigue. For children, symptoms like nausea, vomiting, and diarrhea are common.

“As you can tell, it’s difficult to distinguish flu from other respiratory illnesses on the basis of this mix of science and symptoms alone,” Campbell said.

Because of the similarities between symptoms of a respiratory illness and influenza, there are tests available to differentiate between them. However, Campbell said that in previously healthy patients, a test is likely not necessary.

“Most people who are otherwise healthy and get the flu have mild illness, and they don’t necessarily need to seek medical care,” Campbell said. “However, once flu activity has been identified in the community and people are going to their physician, a clinical diagnosis can often be made just based on the signs and symptoms most consistent with flu.”

Campbell said there are certain at-risk groups, like children and the elderly, that should be tested if possible.

“Tests are most useful when they are likely to yield clinically relevant results that are going to influence clinical management decisions,” Campbell said.

As far as preventing influenza contraction, Campbell said the best defense is the flu vaccine. According to Campbell, interim flu vaccine effectiveness estimates for this season, the overall vaccine effectiveness was 36 percent in February. This effectiveness fluctuates depending on which strain the vaccine is helping to prevent.

"You can be certain that when you get vaccinated, you are receiving protection against multiple viruses."

It is estimated that this year's flu vaccine is 25 percent effective against influenza H3N2, the predominant virus this season. The effectiveness against H1N1 is nearly double that, at 67 percent. The vaccine is also 42 percent effective against influenza B viruses, seen a lot in later part of the season.

As low as some of these numbers can seem, Campbell said they are fairly consistent with past seasons. In addition, the effectiveness on the youth population is much higher, at 51 percent for children 6 months to 8 years old.

“We were very pleased this year with the vaccine effectiveness that we saw among children,” Campbell said.

Campbell said that one of the hypotheses she has for why the vaccine effectiveness is significantly lower for H3N2 is because of how the vaccine is created.

“One idea that has come up in recent years that seems to be playing a part for H3N2 viruses is that when these are optimized for growth in eggs—which is actually required for the production of most US flu vaccines—the viruses develop egg-adapted changes,” Campbell said. “This does occur with a greater frequency in these H3N2 viruses. We think that’s contributing to why the vaccine ends up being less effective because the vaccine viruses were grown up in eggs.”

Other factors that could contribute to vaccine effectiveness include a person's immune system, which can vary depending on when the person was born or what type of flu viruses they previously encountered, whether through the environment or through vaccination.

Campbell said another large factor is simply that the flu virus changes year to year, requiring the vaccine to do the same.

“All flu viruses do have small changes year to year, and that’s why the vaccine needs to be reformulated every season,” Campbell said.

Every year, trivalent and quadrivalent vaccines are manufactured, the first containing the H3N2 virus, H1N1 virus, and one B virus, and the second containing those three plus another B virus. Campbell said this increases the chances of preventing the flu.

“You can be certain that when you get vaccinated, you are receiving protection against multiple viruses," Campbell said. “That’s actually really important because even though vaccine effectiveness may be less than we had hoped for H3N2 viruses, for example, it was still quite effective for H1 and B viruses.”

Despite its benefits, Campbell said she sees far fewer adults and children getting vaccinated than what is recommended, leading to increased risk of infection.

“It’s really most important just to encourage vaccination and make that a very straightforward message,” Campbell said.

The idea of a universal flu vaccine has left many hopeful that flu seasons like this year's may soon be a thing of the past, but Campbell said that is still a long way off.

“A number of government agencies and private companies have begun that work, and we are hopeful, but I think it’s still off in the future,” Campbell said. “I think we have to work with what we have right now.”

In the meantime, the public has a lot of uncertainty and misinformation surrounding influenza. Campbell said she is working to dispel some of the common flu myths. For example, some believe a person can carry the flu without showing any symptoms, thereby passing it on to others.

Campbell said some people who have the flu may not show symptoms and be asymptomatic, but it is still for a time-limited period. Healthy people often have the virus present in them and may be able to shed that virus even a day before their symptoms develop.

Another common fear is that getting vaccinated against the flu will end up infecting the individual rather than preventing such sickness. Campbell said this fear largely comes from coincidence and correlation, not causation.

“One thing to keep in mind is that a lot of people are being vaccinated at a time of year when a lot of respiratory viruses are circulating, and so, it’s not uncommon to become ill with any of those viruses around the time that you received your flu vaccination,” Campbell said. “I think that sometimes confuses the picture a little bit.”

There is also the belief that getting the flu vaccine can decrease one's ability to spread the flu if they were to catch it. Campbell said there is not much evidence to support this, though the vaccine has been shown to make the flu milder for those who contract it. This, she said, could contribute to the length of time for which one is contagious.

“It’s true that vaccination really can give you milder disease, and I would say therefore you’re probably not going to shed virus for as long or at as high levels,” Campbell said.

In all, Campbell said the safest bet is to get vaccinated, and if you do contract the flu, stay home and rest. The best way to prevent another bad flu season, Campbell said, is to decrease the chances of it spreading as much as possible.