Does H1N1 vaccine interfere with HIV tests?


Back in June I had unprotected sex. I also in Sept heavily made out with a girl after a night of drinking...I know based on reading the archives no risk from kissing. On 12 Oct, I took a Home Access Express HIV test that was negative. Being the paranoid person I am, I am still going to take another one after 6 months, even though I believe you would say I am good to go based on the 3 month test. I also just got the H1N1 vaccine. So the question is will the H1N1 vaccine interfere with a future test of HIV specifically the one done by Home Access? I assume they are seperate antibodies that they check for?

Thank you. Your service is great!



Hello J Dog,

This is the first year we are administering the H1N1 vaccine; consequently, we can't give a definitive answer. You are correct that the antibodies involved would be specific for the pathogen. (HIV would produce anti-HIV antibodies and H1N1 vaccine would produce antibodies against the H1N1 virus.) However, it may be possible that H1N1 vaccines could produce some cross-reacting antibodies that might be picked up on an HIV-antibody test, thereby giving a false-positive result. This happens with seasonal flu vaccines (see below). Consequently, it could also occur with H1N1 vaccination. These false-positive results can be quickly identified with the appropriate follow-up tests.

Dr. Bob

Very Confused About the Flu Shot and Testing (FLU VACCINE) Dec 12, 2008

Hi Dr. Bob,

Thanks for your work, your forum is very informative. I have searched and searched everywhere for information about the flu shot reacting with the hiv test. I know its mentioned on your forum, but I couldn't find the answer to my question- I don't even know if there is one!

A few months ago, I made an error in judgement and had unprotected sex with a guy I have known for a long time. I have no reason to believe he was HIV +, but you never know. I do know he sleeps with other women, but I don't know how often or how many. He does not use IV drugs.

I also have a boyfriend who I care very much about. I came clean to him about the mistake and we are trying to work things out. The last step to putting this behind us is me getting an HIV test. I am his only risk factor, as he is not a drug user and has only had sex with me.

The incident happened at the beginning of October. I have a test scheduled for Dec. 30. It will be in the middle of the 13th week after the incident, about 87 days, so I know if it comes back negative it will be conclusive.

My concern is that I had a flu shot when I went to the doctor on Dec. 2. I have read here and other places that there can be some reaction on HIV tests. I want to get this test over with so badly and move on with my life. But I also don't want to take the risk of a false positive test. What are the chances the shot will affect my test and is there any length of time it is recommended to wait before getting the test?

Thank you so much! I'm a first year law student trying to worry about all this and study for finals, a response would really help me out!

Response from Dr. Frascino


I would suggest you get your 13-week test as scheduled. See below for information regarding the flu vaccine and HIV tests.

Dr. Bob


A week ago my 17-year-old daughter received a letter from Red Cross indicating she screened HIV positive after donating at a recent blood drive. Results were HIV-1/HIV-2 Positive, Confirmatory Indeterminate, NAT Negative. Retesting was done at a hospital lab. Results are HIV (weak) negative and we await Confirmatory results. My daughter appears to have no risk factors. She is a virgin, has no history of sexual activity whatsoever, no tattoos, no IV drug use. Ten days before the blood donation she received (her first ever) flu vaccine and also the second of three Gardasil shots. She was also prescribed Provera a month earlier. After reading previous posts I believe you draw no correlation between vaccines and false positive HIV tests, however there does seem to be recent information to the contrary. Information published in March 2006 in the New England Journal of Medicine asserts recent inoculation with any brand of influenza vaccine was significantly associated with a false positive screening assay for HIV antibodies. Guidelines of both Johns Hopkins and the New York State Department of Health list influenza vaccination as a known cause of indeterminate results on Western blotting for HIV antibodies. Given the escalating international awareness of various influenza strains, it is very important to remind patients and clinicians that influenza vaccination may cause cross-reactivity with HIV antibody assays. I'm not qualified to debate the validity of this information, but it is somewhat comforting for a family in a situation like ours to think that a vaccine might be the issue. My question: What is your opinion of the likelihood of an HIV positive confirmation considering my daughters history? Thank you.

Response from Dr. Frascino


Thanks for your post. Yes, we are trying to get this more recent information out to a wider audience. The influenza vaccine changes from year to year to cover the most likely flu strain candidates for the coming flu season. We do have recent information that flu vaccination can cause some degree of nonspecific cross-reacting antibodies, which can confound HIV-antibody tests. Generally speaking, these cross-reacting antibodies cause "indeterminate" rather than "false positive" HIV tests. Please note a reactive ELISA with subsequent indeterminate Western Blot is considered to be an "indeterminate" HIV test result, not a positive test! HIV testing center and certainly HIV specialists are well aware of these potential complicating factors and we have methods for sorting out serological results that are either in dispute or indeterminate.

In your daughter's case of essentially no potential HIV exposure and a recent flu vaccine, you can rest assured her indeterminate Western Blot is not related to HIV seroconversion, but rather to the detection of cross-reacting antibodies. That her NAT was negative confirms this fact, because NAT testing does not rely on antibody detection.

Happiest Holidays to you and your entire family.

Dr. Bob