Vaccines and COVID testing

With the increased roll out of COVID vaccinations, more than 22 million people in the UK alone have had their first injection, there are a number of questions arising relating to testing and the results of the different tests. 

What exactly does the Vaccine do? 

The two vaccines currently being used in the UK work in slightly different ways. 

The Pfizer/BioNTech vaccine contains an artificially generated portion of viral mRNA (messenger ribonucleic acid). This carries the specific genetic instructions for your body to make the coronavirus’s “spike protein”, against which your body mounts a protective immune response. 

The AstraZeneca vaccine uses a different technology. It packages viral DNA into a viral vector or “carrier” from a different virus. The DNA prompts your body to again produce the “spike” protein, which is again targated by your own body’s immune response. 

In both cases your own body is “tricked” into mounting an immune response to the S protein on the virus. The S protein is important in allowing the virus to bind to normal cells – so by neutralising this the vaccines afford a degree of protection. 

Do either of the Vaccines alter the result of tests? 

The short answer is no. 

PCR tests: In the case of the Pfizer/BionTech vaccine although RNA is what is detected by PCR tests, the mRNA used is only a small part of the entire viral RNA. The mRNA injected cannot make copies of itself and also is unstable- being degraded within hours -therefore there would not be sufficient quantity to be detected, even using a PCR test. Similarly the AstraZeneca vaccine only contains a small part of the DNA that cannot replicate so cannot give you infection or a positive PCR test. 

Antigen tests: These vaccines will not cause rapid antigen tests to be positive, since the proteins produced following vaccination are not expressed in the respiratory (i.e., nasal) tract, which is sampled for Covid-19 PCR or antigen testing. Furthermore the tests used by TAC detect the N protein rather than the S protein which are the targets of the vaccines. 

So what does it mean when a vaccinated person tests positive? 

It is important to realise that being vaccinated is not a guarantee that you will not catch COVID or that you cannot pass it on, however the vaccines are likely to reduce the incidence of asymptomatic infection and have demonstrated a high level of efficacy in preventing symptomatic and severe disease. 

If you have recently been vaccinated and test positive for COVID it is likely that you were infected with the virus just prior to or after being vaccinated. Unfortunately, we do not know how long immunity lasts for after vaccinations and any positive test does require to be treated seriously. 

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