US Pharm
. 2023;48(12):36-37.

Influenza, also known as the flu, is a contagious disease that is caused by influenza viruses. Influenza viruses infect the respiratory tract (i.e., nose, throat, and lungs). The flu shot (vaccine) protects people against the influenza virus infection. The CDC recommends that adults receive the annual flu shot in September or October. Children aged younger than 8 years who need two doses of the vaccine should get the first in the series in July or August. There is no set time frame, however, since the flu can begin spreading early in the fall and last through May. This means there is a lot of variation when it comes to the optimal time to get the flu shot.1

The flu shot is not an active vaccine since the viruses in it are killed through a physical or chemical process (inactivated), so people cannot get flu from a flu shot. Some minor side effects that may occur include soreness, skin redness, fever, and minor swelling and pain where the shot was given. There are some known adverse effects of the flu shot, including serious disease processes such as Guillain-Barré syndrome and other recognized side effects such as permanent shoulder injuries related to the administration of the vaccine.1

Flu viruses are constantly changing, and the flu vaccine will normally be updated from one season to the next to protect against the viruses that research suggests will be common during the upcoming flu season. The protection from a flu vaccine reduces over time, and yearly vaccination is needed for the best protection. Flu vaccines have a very good safety profile.1 Receiving a flu shot every flu season is advisable regardless of whether the administrations are 11 or 13 months apart.1,2

The most commonly administered flu vaccine is a quadrivalent vaccine, which provides protection against the four strains of influenza viruses most likely to be circulating each season.1,2 The LAIV4 (egg-based) vaccine has been licensed by the FDA since 2003. The LAIV4 contains live, attenuated influenza viruses. LAIV4 is administered intranasally using the supplied prefilled, single-use sprayer containing 0.2 mL of vaccine.1,2

FDA Recommendations

On March 7, 2023, the FDA’s Vaccines and Related Biological Products Advisory Committee met in Silver Spring, Maryland, to select the influenza viruses for the composition of the influenza vaccine for the 2023–2024 U.S. influenza season. The advisory committee reviewed and evaluated the surveillance data related to epidemiology and antigenic characteristics of recent influenza isolates, serological responses to 2022–2023 vaccines, and the availability of candidate strains and reagents.1,2

Influenza virus strains were selected based on the influenza vaccine production method—egg-based and cell- or recombinant-based.1,2

The committee recommended that the quadrivalent formulation of egg-based influenza vaccines for the U.S. 2023–2024 influenza season contain the following:

• One A/Victoria/4897/2022 (H1N1)pdm09-like virus
• One A/Darwin/9/2021 (H3N2)-like virus
• One B/Austria/1359417/2021-like virus (B/Victoria lineage)
• One B/Phuket/3073/2013-like virus (B/Yamagata lineage).

The committee recommended that the quadrivalent formulation of cell- or recombinant-based influenza vaccines for the U.S. 2023–2024 influenza season contain the following:

• One A/Wisconsin/67/2022 (H1N1)pdm09-like virus
• One A/Darwin/6/2021 (H3N2)-like virus
• One B/Austria/1359417/2021-like virus (B/Victoria lineage)
• One B/Phuket/3073/2013-like virus (B/Yamagata lineage).

For trivalent influenza vaccines for the 2023–2024 influenza season in the U.S.—depending on the manufacturing method of the vaccine—the committee recommended that the A(H1N1)pdm09, A(H3N2), and B/Austria/1359417/2021-like (B/Victoria lineage) viruses recommended above for the quadrivalent vaccines be used.1,3

Health officials plan ahead for flu season before the summer. The CDC make its annual flu shot recommendations for health professionals ahead of the flu season.3

According to the CDC, preliminary estimates show that, last season, people vaccinated against the flu were about 40% to 70% less likely to be hospitalized because of flu illness or related complications.3 The committee also believes that the effectiveness of flu vaccines depends, in part, on the match between the vaccine viruses and circulating viruses.3

CDC Recommendations

This year’s recommendations bring good news for people with egg allergies.4,5 Per the CDC, flu vaccines are produced using an egg-based manufacturing process and contain a small amount of egg proteins such as ovalbumin. Previously, it was recommended that persons with egg-related allergies get their vaccine under the supervision of a healthcare provider who is “able to recognize and manage severe allergic reactions if an egg-based vaccine is used.” This, according to the report, is no longer necessary.

Egg allergy alone necessitates no additional safety measures for influenza vaccination beyond those recommended for any recipient of any vaccine, regardless of severity of previous reaction to egg.4,5

The CDC recommends that people aged 6 months and older get a flu vaccine each year, as long as they do not have certain symptoms or conditions. Individuals who should consult a medical professional about flu vaccines include those who are immunocompromised, those who have a history of severe allergic reaction to any component of the vaccine, children aged 2 to 4 years who have received a diagnosis of asthma, and pregnant females.4,5

It is further suggested in the report that vaccination should continue after October and throughout the given flu season since “influenza activity might not occur in certain communities until February, March, or later.” Additionally, the CDC recommends vaccination “as long as flu viruses pose a threat.”5 The CDC says that the start, peak, and decline of each flu season cannot be predicted, and the ideal time to start vaccination can change.4

For people aged 65 years and older, there are three flu vaccines that are preferentially recommended over other flu vaccines: Fluzone High-Dose Quadrivalent inactivated flu vaccine, FluBlok Quadrivalent recombinant flu vaccine, and Fluad Quadrivalent adjuvanted inactivated flu vaccine.4

Multiple Vaccinations

Several vaccines can protect against respiratory viruses. These viruses include flu, COVID-19, and respiratory syncytial virus (RSV). Both the RSV and COVID-19 vaccines are relatively new.5,6 It is safe to receive a flu and COVID-19 shot at the same time.

Getting flu and COVID-19 vaccines at the same visit is recommended if the person is eligible and timing for each vaccine is right. The two were often administered at the same time during the past two flu seasons; however, according to research done by the FDA and the Centers for Medicare & Medicaid Services in fall 2022, it was reported that the risk of stroke was more prevalent for adults who received a flu and bivalent COVID-19 vaccine within 6 weeks of each other.5-7

In May, the FDA approved the world’s first RSV vaccine to protect adults aged 60 years and older from the worst consequences of RSV—a common respiratory virus that infects the lungs and breathing passages, including nose and throat.5,6 Since it is so new, information on receiving the RSV vaccine at the same time as others, such as the flu and COVID-19 vaccines, is limited and evolving, according to the CDC reports.5,6


1. CDC. Seasonal flu vaccines. August 25, 2023. Accessed November 1, 2023.
2. CDC. Summary: prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2023-24. August 23, 2023. Accessed November 1, 2023.
3. CDC. Information for the 2023-2024 flu season. October 24, 2023. Accessed November 1, 2023.
4. CDC. Flu vaccine and people with egg allergies. August 25, 2023. Accessed November 1, 2023.
5. CDC. Influenza (flu) vaccines. September 30, 2021. Accessed November 1, 2023.
6. Fiore K. Can COVID, flu, and RSV shots be given at the same time? Medpagetoday. August 29, 2023. Accessed November 1, 2023.
7. Twenter P. COVID-19, flu shots might raise stroke risk for older Americans: preprint. Becker’s Hospital Review. October 25, 2023. Accessed November 3, 2023.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

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