Why is it possible to test positive on a PCR after receiving the covid-19 vaccine?

The effectiveness percentages indicates how much the likelihood of developing symptomatic covid-19 is reduced in vaccinated people


On last April 3, Argentina’s President Alberto Fernández announced on his social networks that he had got covid-19 just two months after receiving the Russian vaccine Sputnik V, a fact that was corroborated by a positive PCR test.

Re-infection by covid-19 is possible after receiving the vaccine, because most of covid-19 vaccines —among them, Sputnik V— have been studied paying special attention to how effective they are with respect to the disease progression, independently of whether the person gets infected or not. In other words: what the effectiveness percentages indicate are how much the likelihood of developing symptomatic covid-19 is reduced in vaccinated people compared to unvaccinated people. Although most people will not be infected with the virus, a little percentage will do, although in a less serious way. As José Antonio Navarro-Alonso, specialised in pediatric medicine, expert in covid-19 vaccination and one of the funder members of the Spanish Association of Vaccinology (AEV), pointed to Verificat, “so far, protection against severe disease is around 100%, and against disease of any intensity is around 95%”.

It is for this reason that Fernández, who was vaccinated earlier this year, developed the disease a few months later. And yet, as a mild case, it corroborates what a study of 20,000 people published in The Lancet says about the Russian vaccine: that it has a 100% efficacy against moderate or serious covid, and an efficacy of 92% against mild to moderate symptomatic covid.

The duration of immunity after vaccination is also still unclear. In theory, the lymphocytes from the immune system can preserve the immunologic memory generated after the injection for years, but “it is still early to know its exact duration”, as indicated in a video by Katherine O’Brien, physician, epidemiologist and director of the Immunisation Department, Vaccinations and Biological Products from the World Health Organisation (WHO). Therefore, there is also no 100% certainty that a person will test negative in a PCR after receiving a vaccine.

The vaccine does not infect you

It is different that you can be re-infected if you come into contact with the virus than if the vaccine itself causes a supposedly contagion. This second assumption is false: you cannot test positive in virus-detection tests after receiving the vaccine due to the action of the vaccine. The Centers for Disease Control and Prevention (CDC) from the United States claim on its website that “neither recently licensed vaccines, nor other recommended vaccines, nor those currently in clinical trials in the US, can cause you to test positive on viral tests used to check if you have an ongoing infection”. The CDC refers, for example, to the PCR used by the Argentinian president.

Although the vaccine itself is not capable of making the virus manifest itself in a viral diagnostic test, it is capable of doing so in an antibody test (the tests that indicate whether the body has developed immunity), which makes sense, as the CDC explains on its website: when the vaccine is inoculated, what is sought is the system to develop antibodies, which are what this type of test detects.