# Why did my PCR test turn out negative if I was vaccinated 18 days ago? **URL:** \ **Category:** The Quarantine Zone\ **Tags:** covid-quarantine\ **Created:** [April 16, 2021, 8:32pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165 "2021-04-16T20:32:55Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [April 16, 2021, 8:32pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/1 "2021-04-16T20:32:55Z")
I was vaccinated 18 days ago with Astrazeneca, so I am a little bit immune by now. I still need a second jab, that will be given in about ten weeks time. But I have to travel abroad tomorrow, so I had to do a PCR test today: those are the rules in Europe. The test result was negative. Now I am wondering how this is possible. Astrazeneca is an inactivated virus vaccine (I hope I got this right), so it contains parts of dead viruses. Those virus particles (the spikes?) stimulate my immune system to prepare for the real viruses. When undergoing the test nobody asked if I was vaccinated. So I wonder how does the vaccine work to confer immunity without giving a positive result. Please do not misunderstand me: I am very happy to have been given a jab _and_ a negative result. I just wonder how this is possible. Maybe I have understood the workings of the vaccine or how the test operates wrong? ---
**Author:** ![DMC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dmc/32/18049_2.png) [@DMC](https://boards.straightdope.com/u/DMC)\ **Post date:** [April 16, 2021, 8:36pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/2 "2021-04-16T20:36:46Z")
From [here](https://theconversation.com/will-the-covid-vaccine-make-me-test-positive-for-the-coronavirus-5-questions-about-vaccines-and-covid-testing-answered-155958): > The AstraZeneca vaccine also only contains part of the DNA [but is inserted](https://vk.ovg.ox.ac.uk/vk/covid-19-vaccines) in an adenovirus carrier that cannot replicate so cannot give you infection or a positive PCR test. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [April 16, 2021, 8:43pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/3 "2021-04-16T20:43:03Z")
So a PCR will be negative but an antibody test _could_ be positive? OK, though I had understood that PCR-test were extremely sensitive: they replicate minuscule bits of DNA or RNA until they are detectable. But I am glad to read that vaccines do not invalidate tests. I firmly believe tests are an important element in the fight against Covid-19. ---
**Author:** ![IvoryTowerDenizen](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ivorytowerdenizen/32/17937_2.png) [@IvoryTowerDenizen](https://boards.straightdope.com/u/IvoryTowerDenizen)\ **Post date:** [April 16, 2021, 9:28pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/4 "2021-04-16T21:28:32Z")
The sensitivity of the of the test isn’t the issue. Antibodies are produced as a response to infection or vaccination. So, vaccination will appear like you’ve had the illness. PCR test detects the presence of active virus, which vaccination does not cause. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [April 16, 2021, 9:33pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/5 "2021-04-16T21:33:07Z")
> [@IvoryTowerDenizen](#): > > PCR test detects the presence of active virus Do you know how PCR distinguishes between active and inactive viruses? Both are RNA strands, what is the difference? ---
**Author:** ![IvoryTowerDenizen](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ivorytowerdenizen/32/17937_2.png) [@IvoryTowerDenizen](https://boards.straightdope.com/u/IvoryTowerDenizen)\ **Post date:** [April 16, 2021, 9:34pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/6 "2021-04-16T21:34:38Z")
Generally you need active virus to get the amount of virus high enough to a level that is detectable. But you are correct that the nuclei acid detected is not only from actively replicating viruses. The larger point is that PCR test detects the infection itself while the antibody test detects the body’s response to being exposed to the antigen, either by infection or vaccination. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [April 16, 2021, 9:44pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/7 "2021-04-16T21:44:15Z")
So the cheaper antibody tests will become useless as more people get vaccinated and PCR test will not. OTOH all tests will be superfluous when enough people get immune through vaccination, but this still complicates the matter. Hmmm… ---
**Author:** ![IvoryTowerDenizen](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ivorytowerdenizen/32/17937_2.png) [@IvoryTowerDenizen](https://boards.straightdope.com/u/IvoryTowerDenizen)\ **Post date:** [April 16, 2021, 9:47pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/8 "2021-04-16T21:47:08Z")
The antibody tests already had limited utility since antibody level naturally declines in months after vaccination or illness. Antigen tests, which most rapid tests were, will still be useful to detect infection. Vaccinations shouldn’t interfere with those. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [April 16, 2021, 9:49pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/9 "2021-04-16T21:49:20Z")
Could you explain what the difference between antigen and antibody tests is? ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [April 16, 2021, 9:50pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/10 "2021-04-16T21:50:00Z")
> [@Pardel-Lux](#): > > they replicate minuscule bits of DNA or RNA until they are Replication of _specific fragments_ of DNA/RNA. Then the solution is tested for the presence of fragments of the _specific length_ of the specific fragments. Even if you had the vaccine misted onto a swab, it still wouldn’t show up on a PCR test unless the ‘primer’ used for the PCR test matched the DNA fragment used for the vaccine. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [April 16, 2021, 9:55pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/11 "2021-04-16T21:55:17Z")
That sounds logical. I knew this was the right place to ask, thank you all very much! ---
**Author:** ![IvoryTowerDenizen](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ivorytowerdenizen/32/17937_2.png) [@IvoryTowerDenizen](https://boards.straightdope.com/u/IvoryTowerDenizen)\ **Post date:** [April 16, 2021, 10:05pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/12 "2021-04-16T22:05:14Z")
Sure. Antigens are portions of the virus the body recognizes as a foreign invader. When the body sees the antigen it mounts an immune system response to destroy the invader and to learn, so it responds even faster and stronger the next time. When the virus is actively infecting there are lots of antigens present (since they are parts of the virus itself). Antigen tests look directly for those virus parts and react to them directly. You need to have a lot of virus. The antigen signal is amplified and made visible using a dye that results in a color changes a lot like a rapid strep test or even a pregnancy stick test. PCR tests also detect virus directly, by recognizing the RNA in the virus. The test amplifies the RNA through the process called PCR, so that it is detectable. This is a much more sensitive test (less starting virus needed) and highly specific (possibility the antigen test could cross react with antigens from harmless sources). Antibodies are the body’s response to an infection when it’s fighting it off. So, it’s only present in high enough levels while actively fighting an infection off (or post vaccination ), and a little after after the infection has cleared. But then the body makes memory cells, to produce antibodies at high levels, rapidly, should another exposure to the virus occur. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [April 16, 2021, 10:13pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/13 "2021-04-16T22:13:51Z")
Beware: when reading reports on the WWW, I have sometimes seen antigen and antibody tests confused. Antibody tests, as described above, test for antibodies, _and they do so by using antigens_. Antigens are the things antibodies search for and destroy: one way to detect antibodies is to put an antigen (or a set of antigens) on a test card, or in a test solution, and see if anything binds to the card/solution. ---
**Author:** ![IvoryTowerDenizen](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ivorytowerdenizen/32/17937_2.png) [@IvoryTowerDenizen](https://boards.straightdope.com/u/IvoryTowerDenizen)\ **Post date:** [April 16, 2021, 10:23pm UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/14 "2021-04-16T22:23:34Z")
Excellent point. And I’ve heard medical professionals mistake antigen and antibody, by a slip of the tongue. You have to be very careful! ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [April 17, 2021, 12:20am UTC](https://boards.straightdope.com/t/why-did-my-pcr-test-turn-out-negative-if-i-was-vaccinated-18-days-ago/940165/15 "2021-04-17T00:20:47Z")
> [@Pardel-Lux](#): > > So the cheaper antibody tests will become useless as more people get vaccinated and PCR test will not. Antibody tests use _specific antigens_ which test for _specific antibodies_, which are reactive to _specific sites_ on the virus. They are quite narrow. The mRNA vaccines are very narrow: they excite only a very narrow range of antibodies. The virus itself can, and probably does, lead to the generation of different antibodies. Thus, different antibody tests can show the difference between vaccine-induced immunity and disease induced immunity. It is possible to test in a pathology laboratory. If it becomes important, rapid tests will be characterized. A test which uses a large range of different antigens will always be slightly more expensive and slightly more difficult to interpret, but not necessarily significantly so. A test that only uses one antigen might be defeated by a simple COVID mutation.