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Flublok® (Influenza Vaccine)

Safety Profile

Flublok has a well-established safety profile1

Flublok is a vaccine indicated for active immunization for the prevention of disease caused by influenza A virus subtypes and influenza type B virus represented by antigens contained in the vaccine. Flublok is approved for use in individuals 9 years of age and older. 

On this page, you will learn:

  • Flublok has a comparable safety profile to a standard-dose quadrivalent inactivated influenza vaccine in adults aged 50+2

  • There were no significant observed differences in safety profile between the 9 to 17-year-old and 18 to 49-year-old populations1

  • In patients aged 18-49, systemic symptoms following vaccination were similar between people receiving Flublok and placebo3

  • Flublok has a similar safety profile compared to standard-dose flu vaccines in adults 50-641

Safety from pivotal trials

Similar safety profile compared to standard-dose flu vaccines in adults 50-641

Safety In Adults Aged 50-64: Reactions, Any Grade

  • Rates of local and systemic adverse reactions were similar within 7 days of Flublok (trivalent) or standard-dose influenza vaccine administration1
  • Pooled data from 2 studies; comparators were Fluzone (trivalent standard-dose formulation) and Afluria (trivalent standard-dose formulation)1
Bar chart comparing side effects of Flublok vs standard-dose flu vaccines

Safety profile in adults aged 18-49 compared to placebo3

Safety Trial in Adults Aged 18-49:

  • Safety data from a study of 4648 adults randomized to receive Flublok (trivalent; n=2344) or placebo (n=2304)3
  • Systemic symptoms following vaccination were similar between people receiving Flublok and placebo3
  • The most frequently reported systemic symptoms following vaccination were headache (15% with Flublok vs 15% with placebo) and fatigue (15% with Flublok vs 15% with placebo)3
    • 76% of headache complaints were mild3
  • Flublok was associated with local injection-site pain (37% with Flublok vs 8% with placebo) that was significantly more frequent than after saline placebo1,3
    • 94% of all pain complaints after Flublok were rated as mild3
Bar chart comparing side effects of Flublok vs placebo

Safety profile in children aged 9-171:

  • There were no significant observed differences in safety profile between the 9 to 17-year-old and 18 to 49-year-old populations
  • Most common adverse reactions in 9 to 17-year-olds (N=608-618):
    • Injection-site reaction: pain (34.4%)
    • Systemic reactions: myalgia (19.3%), headache (18.5%), malaise (16.1%)

Real-world safety evidence

Safety profile supported in one of the largest safety studies of a flu vaccine in pregnant women, with ~15,000 patients1,4,5

Study Design

  • This was an observational, retrospective safety surveillance study of Flublok Quadrivalent in 14,981 pregnant individuals across Northern Hemisphere influenza seasons 2018-2019 and 2019-20201*†‡
  • This study represents one of the largest populations of pregnant people to be included in the prescribing information for any FDA-approved flu vaccine1,4,5
Baby in womb icon

Flublok demonstrated no increased risk of major birth defects and miscarriages1

These data support the safety profile of Flublok.1

*5842 patients, including those with chronic conditions, were exposed to Flublok Quadrivalent during the 28 days prior to conception or during the first trimester. Miscarriage was reported in 464 (3.1%) patients. Among individuals exposed to Flublok Quadrivalent at any time during pregnancy, 1113 pregnancies (7.7%) had infants with major birth defects (56, 360, 381, and 316 among individuals exposed during the 28 days prior to conception, the first trimester, the second trimester, and the third trimester, respectively).1    
†Prespecified outcomes included spontaneous abortion and congenital/fetal anomalies. Data were not collected on ectopic pregnancy or elective terminations.1    
‡The data for Flublok (quadrivalent) are relevant to Flublok (trivalent) because both vaccines were manufactured using the same process and have overlapping compositions.1

The CDC recommends flu vaccination for pregnant people to help protect them and their babies6

  • Pregnant people are at higher risk of severe flu-related illness and hospitalization7
  • Pregnant people who receive a flu vaccination:
    • Have 40% lower risk of hospitalization with flu8§
    • Help protect their babies from flu-related illness and hospitalization when they are too young to be vaccinated6


§Retrospective test negative design study, 2010-2016, which evaluated adjusted overall influenza vaccine efficacy against influenza-associated hospitalization during pregnancy (N = 1030).8

Frequently asked questions

The Flublok single-dose syringes contain no natural rubber latex.1

Flublok is stored refrigerated between 2° and 8°C (36° and 46°F).1

For resources to help support your patients, including the Flublok patient brochure, please visit our Resources section.

For Flublok, in children 9 through 17 years of age who received Flublok Quadrivalent, the most common solicited injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were myalgia, headache, and malaise. In adults 18 through 64 years of age who received Flublok, the most common injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were headache, fatigue, and myalgia. In adults 65 years of age and older who received Flublok, the most common injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were fatigue and headache.1

In an analysis of data on ~15,000 pregnant individuals, including those with chronic conditions, who were exposed to Flublok during the 28 days prior to conception or throughout their pregnancy, data showed no increased risk of miscarriages or major birth defects with vaccine exposure.1,4,5       

The study was conducted during the 2018-2019 and 2019-2020 flu seasons and assessed specific pregnancy outcomes; not all possible impacts were assessed.1,4,5       

Talk to your pharmacist or doctor to find out if Flublok is right for you or if you have more questions. Healthcare providers are encouraged to enroll women who receive Flublok during pregnancy in Sanofi’s vaccination pregnancy registry by calling 1-800-822-2463 or going to https://www.sanofivaccinespregnancyregistry.com.

No, Flublok does not contain thimerosal.1

Flublok® Influenza Vaccine and VaccineShop Logo

Order Flublok

To order Flublok, call 1-800-VACCINE (1-800-822-2463) or click below to order on VaccineShop

Order on VaccineShop
Fluzone® High-Dose Influenza Vaccine Logo

About Fluzone High-Dose

A high-dose flu vaccine for your patients 65+9

Learn more about Fluzone High-Dose
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The Burden of Influenza

It's only the flu, until it isn't

Find out more about the complications of flu
Important Safety Information

Do not administer Fluzone, Flublok, or Fluzone High-Dose to anyone with a history of a severe allergic reaction (e.g., anaphylaxis) to any component of the vaccine (including egg protein for Fluzone and Fluzone High-Dose). Fluzone and Fluzone High-Dose should not be administered to anyone who has had a severe allergic reaction after previous dose of any influenza vaccine.

Appropriate medical treatment must be immediately available to manage potential anaphylactic reactions following administration of the vaccine.

If Guillain-Barré syndrome has occurred within 6 weeks following previous influenza vaccination, the decision to give Fluzone, Flublok, or Fluzone High-Dose should be based on careful consideration of the potential benefits and risks.

Syncope (fainting) has been reported following vaccination with Fluzone, Flublok and Fluzone High-Dose. Procedures should be in place to avoid injury from fainting.

In two separate postmarketing observational studies, an increased risk of febrile seizures was observed during the first day following vaccination with standard dose trivalent (2024-2025) and quadrivalent (2023-2024) influenza vaccines in children 6 months through 4 years of age.

If Fluzone, Flublok, or Fluzone High-Dose are administered to immunocompromised persons, including those receiving immunosuppressive therapy, the expected immune response may not be attained.

Vaccination with Fluzone, Flublok, or Fluzone High-Dose may not protect all recipients.

For Fluzone, in children 6 months through 8 years of age, the most common injection-site adverse reactions were pain or tenderness and redness; the most common solicited systemic adverse reactions were irritability, drowsiness (6 months through 35 months), and myalgia (3 years through 8 years). In adults 18 through 64 years of age, the most common injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were headache and myalgia. In adults over 65 years of age, the most common injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were headache, myalgia, and malaise.

For Flublok, in children 9 through 17 years of age who received Flublok Quadrivalent, the most common solicited injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were myalgia, headache, and malaise. In adults 18 through 64 years of age who received Flublok, the most common injection site adverse reaction was pain; the most common solicited systemic adverse reactions were headache, fatigue, and myalgia. In adults 65 years of age and older who received Flublok, the most common injection-site adverse reaction was pain; the most common solicited systemic adverse reactions were fatigue and headache.

For Fluzone High-Dose, in adults 65 years of age and older, the most common injection-site reaction was pain; the most common solicited systemic adverse reactions were myalgia, malaise, and headache.

For Fluzone, Flublok, and Fluzone High-Dose, other adverse reactions may occur.

Indications

Fluzone, Flublok, and Fluzone High-Dose are vaccines indicated for active immunization for the prevention of disease caused by influenza A virus subtypes and type B virus contained in (or in the case of Flublok, represented by antigens contained in) the vaccine. Fluzone is approved for use in persons 6 months of age and older. Flublok is approved for use in individuals 9 years of age and older. Fluzone High-Dose is approved for use in persons 65 years of age and older.

Before administration, please see the full Prescribing Information for Fluzone, Flublok, or Fluzone High-Dose. Also, please see complete Patient Information for Fluzone or Fluzone High-Dose.

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Important Safety Information
Indications

References: 1. Flublok. Prescribing Information. Protein Sciences Corporation. 2. Dunkle LM, Izikson R, Patriarca P, et al; PSC12 Study Team. N Engl J Med. 2017;376(25):2427-2436. doi:10.1056/NEJMoa1607762 3. Treanor JJ, El Sahly H, King J, et al. Protective efficacy of a trivalent recombinant hemagglutinin protein vaccine (FluBlok®) against influenza in healthy adults: a randomized, placebo-controlled trial. Vaccine. 2011 Oct 13;29(44):7733-9. doi: 10.1016/j.vaccine.2011.07.128 4. Fluarix. Prescribing Information. GlaxoSmithKline. 5. Flucelvax. Prescribing Information. Seqirus Inc. 6. Centers for Disease Control and Prevention. Flu vaccine safety and pregnancy. Published September 17, 2024. Accessed October 14, 2024. 7. Grohskopf LA, Ferdinands JM, Blanton LH, et al. MMWR Recomm Rep. 2024;73(5):1-25. doi:10.15585/mmwr.rr7305a1 8. Thompson MG, Kwong JC, Regan AK, et al; PREVENT Workgroup. Clin Infect Dis. 2019;68(9):1444-1453. doi:10.1093/cid/ciy737 9. Fluzone High-Dose. Prescribing Information. Sanofi Pasteur Inc.

MAT-US-2607248-v1.0-08/2026 Last updated: August 2026

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