Category Archives: Q&A

Time period after receiving influenza vaccine that a nasal swab can give a false positive result?

Q: We have had three employees who have tested positive for influenza by nasal swab within two weeks of receiving Fluarix Quadrivalent (GlaxoSmithKline) vaccine. Is there a time period after receiving influenza vaccine that a nasal swab can give a false positive result?

A: Inactivated influenza vaccines, including Fluarix, are not known to cause false positive nasal swab tests. However, false positive test results are possible with rapid tests, and these are more likely to occur when influenza prevalence in the area is low. For more information regarding interpretation of rapid influenza tests, see www.cdc.gov/flu/professionals/diagnosis/rapidlab.htm.

Q: A healthcare worker with no history of chickenpox, and unknown serologic immunity, was exposed to a patient with zoster. She received varicella vaccine two days later. She developed a pruritic maculopapular rash 11 days after vaccination. Is the rash from the vaccine or from her zoster exposure?

A: The only way to determine whether the rash is caused by wild-type varicella or vaccine virus is to try to isolate virus from the rash and send it to a laboratory that is capable of differentiating wild and vaccine-type virus. This is generally not practical. Given the history, the conservative approach is to assume she has an active case of chickenpox and act according to your infection control guidelines.

Q: I have a patient who is now 12 weeks pregnant. In September 2016, she received quadrivalent inactivated influenza vaccination (before she was pregnant). Should we give her another dose of 2016–17 influenza vaccine since she was not pregnant at the time of her first dose?

A: The Advisory Committee on Immunization Practices (ACIP) does not recommend more than one dose of influenza vaccine per season, except for certain children being vaccinated for the first time. The 2017–18 influenza vaccine may be available near the end of her pregnancy, so she can be given a dose of next year’s formulation at that time.

Q: If a person received a Tdap vaccine and then had a positive pertussis PCR two weeks later, could it be a false positive from the vaccine, or should we consider this a case of pertussis? The patient had a cough, nausea and vomiting for two to three days prior to PCR testing.

A: Recent Tdap vaccination does not affect PCR testing. PCR tests are used to detect DNA sequences of the Bordetella pertussis bacterium. PCR tests are very sensitive and could give a false positive result for other reasons. For more information on the interpretation of pertussis diagnostic tests, see
www.cdc.gov/pertussis/clinical/diagnostic-testing/diagnosis-confirmation.html.

Q: If a patient receives hepatitis B vaccine while undergoing hemodialysis, will the vaccine be effective? Will the dose need to be repeated?

A: Neither the ACIP nor the manufacturers address the timing of vaccination and dialysis. Persons with end-stage renal disease including predialysis, hemodialysis, peritoneal dialysis and home dialysis should be tested for hepatitis B surface antibody (anti-HBs) one to two months after vaccination, and annually. If the anti-HBs level is below 10mIU/mL, they should be revaccinated. See www.cdc.gov/mmwr/PDF/rr/rr5516.pdf, page 27, for more information.