Flu Season 2020: How well does the flu shot work, is it effective against all types of influenza viruses?
The seasonal flu vaccine is designed to protect against the three or four influenza viruses that research indicates are most likely to spread and cause illness among people during an upcoming flu season. There are many different flu viruses and they are constantly changing. Accordingly, the composition of US flu vaccines are reviewed each year and updated as needed based on which influenza viruses are making people sick, the extent to which those viruses are spreading, and how well the previous season’s vaccine protects against those viruses. The 2020-21 season’s flu vaccines were also updated to better match viruses expected to circulate in the US.
Every flu season, the US Centers for Disease Control and Prevention (CDC) conducts studies to measure the benefits of seasonal flu vaccination to understand how well flu vaccines are working, that is its ability to prevent flu illness. These vaccine effectiveness studies assess and confirm the value of flu vaccination as a public health intervention. While vaccine effectiveness can vary, recent analyses reveal that flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well-matched to the flu vaccine.
According to CDC estimates, influenza vaccination prevented an estimated 7.52 million illnesses, 3.69 million medical visits, 105,000 hospitalizations, and 6,300 deaths during the 2019-20 season. In general, current flu vaccines tend to work better against influenza B and influenza A(H1N1) viruses and offer lower protection against influenza A(H3N2) viruses. “While determining how well a flu vaccine works is challenging, in general, recent studies have supported the conclusion that flu vaccination benefits public health,” says the agency.
While all influenza viruses undergo frequent genetic changes, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses compared with influenza A(H1N1) and influenza B viruses. That means that between the time when the composition of the flu vaccine is recommended and the flu vaccine is delivered, H3N2 viruses are more likely than H1N1 or influenza B viruses to have changed in ways that could impact how well the flu vaccine works.
What factors influence how well the vaccine works?
How well the flu vaccine works can vary from season to season. The vaccine’s effectiveness can also vary depending on who is being vaccinated. At least two factors play an important role in determining the likelihood that the flu vaccine will protect a person from flu illness. This includes characteristics of the person being vaccinated (such as their age and health) and the similarity between the flu vaccine and the flu viruses spreading in the community.
During years when the flu vaccine is not well-matched to circulating influenza viruses, little or no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, it is possible to measure substantial benefits from flu vaccination in terms of preventing flu illness and complications.
Does flu vaccine effectiveness vary by type or subtype?
Yes, it does. According to experts, the amount of protection provided by flu vaccines may vary by influenza virus type or subtype even when recommended flu vaccine viruses and circulating influenza viruses are well-matched.
A study that examined the number of vaccine effectiveness estimates from 2004-2015 found average vaccine effectiveness of 33% against illnesses caused by H3N2 viruses, compared with 61% against H1N1 and 54% against influenza B virus illnesses. Vaccine effectiveness estimates were lower when vaccine viruses and circulating viruses were different.
Is the flu vaccine effective against all types of flu viruses?
Most flu vaccines in the US protect against four different (quadrivalent) flu viruses (quadrivalent): an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses. Some flu vaccines protect against three different (trivalent) flu viruses: an influenza A (H1N1) virus, an influenza A (H3N2) virus, and one influenza B virus.
Flu vaccines do not protect against infection and illness caused by other viruses that can also cause flu-like symptoms. Non-flu viruses that can result in flu-like illness and spread during the flu season include rhinovirus (one cause of the common cold) and respiratory syncytial virus (RSV), the most common cause of severe respiratory illness in young children, and a major cause of severe respiratory illness in adults aged 65 years and older.
Is the flu vaccine effective in the elderly?
Flu vaccines have helped protect adults 65 years of age and older against influenza A(H1N1) viruses and both lineages of influenza B viruses. Specifically, flu vaccines have reduced the risk of “medically attended illness” caused by H1N1 or B by more than 60% on average among people aged 65 and older. Flu vaccines have reduced the risk of flu hospitalization among adults age 65 and older by 54% against A(H1N1) viruses and by 31% against influenza B viruses on average. However, protection against influenza A(H3N2) flu viruses has been less consistent. On average, flu vaccines have reduced the risk of doctor visits with A(H3N2) flu by 24% and lowered the risk of hospitalization with A(H3N2) flu by 33% in adults age 65 and older.
Is the flu vaccine effective in children?
In most seasons, vaccination has been found to provide a similar level of protection against flu illness in children to that seen among healthy adults. In several studies, flu vaccine effectiveness was higher among children who received two doses of flu vaccine the first season that they were vaccinated (as recommended) compared to “partially vaccinated” children who only received a single dose of flu vaccine. However, in some seasons, partially vaccinated children still receive some protection. A recent analysis also shows that even in a season where vaccine components were not well-matched, flu vaccination prevented moderate to severe illness in children.
Different flu shots
While there are many vaccine options to choose from, the CDC does not recommend one flu vaccine over another. The most important thing is that everyone, 6 months and older, should get an age-appropriate flu vaccine yearly, ideally by the end of October, suggests the agency. It takes about two weeks after vaccination for antibodies that protect against flu to develop in the body.
For 2020-2021, the CDC recommends use of any licensed, age-appropriate flu vaccine as an option for vaccination. These include injectable flu vaccines, or live attenuated influenza vaccines, or nasal spray.
Who should not get a flu shot?
Factors that can determine a person’s suitability for vaccination include a person’s age, health (current and past) and any relevant allergies. Children younger than 6 months of age are too young to get a flu shot. People with severe, life-threatening allergies to flu vaccine or any ingredient in the vaccine, such as gelatin or antibiotics, should not receive a vaccine. Find more information regarding who cannot get a flu vaccine and who should talk to their doctor before getting one here.
Vaccine distribution
Manufacturers have started distributing the flu vaccine and will continue to distribute it throughout the season. An estimated 139.4 million doses of flu vaccine have been already distributed. The CDC recommends getting a flu vaccination in September or October but getting vaccinated anytime during the flu season can help protect you. Use the Vaccine Finder to find out where to get vaccinated near you.