What are the potential complications after RSV (Respiratory Syncytial Virus) and flu (influenza) vaccine administration?

Complications After RSV and Flu Vaccines

Both RSV and influenza vaccines are generally well-tolerated with predominantly mild-to-moderate local and systemic reactions, and serious adverse events are rare.

Common Local Reactions

Injection site reactions are the most frequent complications:

• Pain at the injection site occurs in 10-64% of patients after influenza vaccination and typically resolves within 2 days 1
• Redness, swelling, and injection-site edema affect 10.8-15.8% of recipients 1, 2
• When coadministering RSV and influenza vaccines, local reactions remain mostly mild or moderate 3
• Administer vaccines in different limbs when giving multiple vaccines that may cause injection site reactions 1

Systemic Reactions

Fever and constitutional symptoms are common but self-limited:

• Fever begins 6-12 hours post-vaccination and persists 1-2 days 4, 5
• Age-specific fever rates after influenza vaccine: 11.5% (ages 1-5 years), 4.6% (ages 6-10 years), 5.1% (ages 11-15 years) 1, 4
• High-risk children aged 6 months-4 years may experience fever rates up to 27% 1, 4
• Fatigue (23%), myalgia (12%), and headache (9%) are the most common general symptoms after coadministration 6
• Malaise and myalgia typically resolve within 1-2 days 5

Fever management approach:

• Acetaminophen 10-15 mg/kg every 4-6 hours as needed for fever or discomfort 4
• Prophylactic acetaminophen is NOT recommended routinely, only for those with history of seizures or family history of convulsions 4
• Fever ≥40.5°C (≥105°F) within 48 hours constitutes a severe reaction requiring VAERS reporting 4

Serious Adverse Events

Severe complications are rare but require immediate recognition:

• Only 14.2% of all VAERS reports describe serious adverse events (death, life-threatening illness, hospitalization, permanent disability) 2
• Immediate allergic reactions (hives, angioedema, allergic asthma, anaphylaxis) rarely occur and likely result from residual egg protein in influenza vaccines 1
• Persons with documented IgE-mediated hypersensitivity or history of anaphylaxis to eggs should NOT receive influenza vaccine 1

Guillain-Barré Syndrome (GBS) considerations:

• The 1976 swine influenza vaccine was associated with <10 cases per 1 million vaccinated 1
• Evidence for causal relationship with subsequent influenza vaccines is unclear 1
• Neurologic complications should be reported to VAERS even without confirmed causality 1, 7

RSV Vaccine-Specific Considerations (Nirsevimab)

Nirsevimab has specific contraindications and precautions:

• Contraindicated in persons with history of severe allergic reaction (anaphylaxis) after previous dose or to product component 1
• For children with increased bleeding risk, follow ACIP general best practice guidelines for immunization 1
• Adverse reactions after nirsevimab alone should be reported to FDA MedWatch 1
• When coadministered with vaccines, report adverse reactions to VAERS and specify nirsevimab receipt on the form 1

Coadministration Safety Profile

Simultaneous RSV and influenza vaccination is safe:

• Coadministration of RSVpreF with seasonal influenza vaccine demonstrated acceptable safety and tolerability in adults ≥65 years 3
• No vaccine-related serious adverse events were reported in coadministration studies 3
• Grade 3 reactions occurred in only 6% of high-risk heart failure patients receiving both vaccines simultaneously 6
• No exacerbations of heart failure, hospitalizations, or deaths occurred within one week post-vaccination in high-risk patients 6

Critical Pitfalls to Avoid

Do not attribute all post-vaccination symptoms to the vaccine:

• Respiratory or other symptoms after vaccination often represent coincidental illnesses unrelated to the vaccine 5, 7
• Influenza vaccine contains only inactivated, non-infectious viruses and cannot cause influenza 5
• Symptoms persisting beyond 2-3 days with increasing severity require evaluation for alternative causes 7

Reporting Requirements

Healthcare professionals must report specific events:

• Report ALL clinically significant adverse events to VAERS, even without certainty of causation 1, 4
• Any adverse event resulting in hospitalization, disability, or death requires VAERS reporting 4
• For nirsevimab adverse reactions with coadministered vaccines, report to VAERS only (not MedWatch separately) 1