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Side Effects


The most common ( ≥ 10%) injection-site reaction was pain ( > 37%); the most common ( ≥ 10%) solicited systemic adverse reactions were headache ( > 15%), fatigue ( > 15%) and myalgia ( > 11%).

Clinical Trials Experience

Because clinical studies are conducted under widely varying conditions, adverse reaction rates observed in the clinical studies of a vaccine cannot be directly compared to rates in the clinical studies of another vaccine and may not reflect the rates observed in clinical practice.

Flublok has been administered to and safety data collected from 2497 adults 18 through 49 years of age enrolled in two randomized, placebo-controlled clinical trials (1, 2). In both studies local (injection site) and systemic adverse reactions were solicited with the use of a memory aid for 7 days following vaccination. Unsolicited adverse reactions were collected for 28 days post-vaccination and Serious Adverse Events (SAEs) were collected for 6 months post-vaccination via clinic visit or telephone follow up on Day 28, telephone follow up on Day 180, or by spontaneous reporting.

In a clinical trial (Study 1, Table 1) that included 2,344 subjects randomized to receive Flublok and 2,304 subjects randomized to receive saline placebo, the mean age of participants was 32.5 years, 59% were female, and 67% were Caucasian (see Clinical Studies).

Table 1: Frequency of Solicited Injection Site Reactions and Systemic Adverse Reactions within 7 Days of Administration of Flublok or Placebo in Adults 18-49 Years of Age, Study 1, Total Vaccinated Cohort1,2,3

Local Adverse Reactions % %
  Pain 37 8
  Redness 4 2
  Swelling 3 2
  Bruising 3 3
Systemic Adverse Reactions % %
  Headache 15 16
  Fatigue 15 14
  Muscle Pain 11 7
  Fever‡ < 1 < 1
  Joint pain 4 4
  Nausea 6 5
  Chills 3 3
NOTE: Data based on the most severe response reported by subjects. Results > 1% reported to nearest whole percent; results > 0 but < 1 reported as < 1%.
‡ Fever defined as ≥ 99.8°F (37.7°C).
1 Total Vaccinated Cohort is defined as all randomized subjects who received study vaccine according to the treatment actually received and who provided data.
2 Study 1 is registered as NCT00539981 under the National Clinical Trials registry.
3 Denominators for Study 1: The total number of enrolled subjects was 2344 in the Flublok group and 2304 in the placebo group. For all categories except fever, the number of subjects with missing values was 72 in the Flublok group and 73 in the Placebo group so that these denominators are 2272 and 2231 respectively. For fever, 89 Flublok recipients and 104 Placebo recipients were missing data, making these denominators 2255 and 2200 respectively.

Across trials, through 6 months post vaccination, two deaths were reported, one in a Flublok recipient and one in a placebo recipient. Both deaths occurred more than 28 days following vaccination and neither was considered vaccine-related. SAEs were reported by 32 Flublok recipients and 35 placebo recipients. One SAE in a Flublok recipient was assessed as possibly related to the vaccine: pleuropericarditis with effusions requiring hospitalization and drainage. No specific cause was identified. The patient recovered.

In Study 1, the most frequent unsolicited adverse events, occurring in 1%-2% of subjects, were nasopharyngitis, upper respiratory infection, headache, cough, nasal congestion, pharyngolaryngeal pain, and rhinorrhea.

Postmarketing Experience

No postmarketing safety data are available for Flublok.


1. Treanor JJ, El Sahly HM, 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, Vol. 13, 29, pp. 7733-7739.

2. Treanor JJ, Schiff GM, Hayden FG, Safety and immunogenicity of a baculovirus-expressed hemagglutinin influenza vaccine: a randomized controlled trial. JAMA. 2007, Vol. 297, pp. 1577-1582.

Read the Flublok (influenza vaccine for intramuscular injection) Side Effects Center for a complete guide to possible side effects


Data evaluating the concomitant administration of Flublok with other vaccines are not available.

Last reviewed on RxList: 11/25/2013
This monograph has been modified to include the generic and brand name in many instances.

Side Effects

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