PCR or antigen tests do not detect if someone has been vaccinated

They calculate the presence of covid-19 in the body and not the amount of antibodies

4 min
Health personnel doing antigen tests

BarcelonaA PCR or an antigen test detects the presence of covid-19 in the body. The former amplifies the genetic material of the virus and is so sensitive that it can detect it even in case of a very small viral load. The antigen test, on the other hand, only detects a part of the virus protein and, since the signal is not amplified in this case, it is only conclusive when the person has a large amount of the virus. What neither of the two diagnostic tests do is measure the level of antibodies and, therefore, neither a PCR nor an antigen test would be able to know, for example, if a candidate for the presidency of the Generalitat has been vaccinated.

Salvador Illa's refusal to take the tests that TV3 asked him to take for Tuesday's debate has sparked a great rumorology, especially in social networks. Even the president of ERC, Oriol Junqueras, has joined the insinuations. He has done so in the morning in the program Espejo Público of Antena 3 and, in the rally of the Republicans that took place in the afternoon, he has been even more forceful: "Why did he refuse to take a test to ensure the health of all those who participated in this debate? Did the minister take advantage of his position to get vaccinated as the military leadership of the Spanish army did? Catalans have the right to know before casting their vote at the ballot box".

However, neither Illa - the PSC has already denied that he has been vaccinated - nor any other candidate was offered a rapid antibody test or a serology. And even in this case the result would not be conclusive. On Wednesday, the socialist candidate justified not having taken the test because the Regional Ministry of Health recommends doing so only when there is a close contact who has tested positive or when there are symptoms of covid-19. "The PCR test has to be done with a protocol, it is not a whim, and the first thing I have to do is to be consistent with what I defended when I was Minister of Health", he said.

"Vaccines, neither in the Pfizer or Moderna ones, do not inject you virus, but fragments of DNA or RNA material of one of the proteins that surround it (Spike). Therefore, it is not possible to detect viruses, but after a few days you can detect antibodies", Beatriz Mothe, a medical expert in vaccines and immunology at the Germans Trias i Pujol Hospital, has explained. AstraZeneca's vaccine also uses the genetic material but transports it encapsulated in a viral vector, which has been stripped of its pathogenic capacity. For this reason, Gemma Moncunill, an immunologist at the Barcelona Institute for Global Health (ISGlobal), a centre promoted by the Fundació La Caixa, says that with the vaccines available in Spain it is "impossible" to detect in a PCR or antigen test whether a person has been vaccinated. "They are done through a nasal swab; therefore, in the respiratory tract. The vaccine is pricked in the arm and has a local effect: it enters the cells to produce the protein and generate defences", the researcher explains.

Thus, it would be necessary to measure the antibodies. But even in this case it would not be possible, for the time being, to distinguish whether someone has been vaccinated. "Neither a rapid antibody test nor serology can differentiate between someone who has passed the covid test and someone who has been vaccinated. In the future they might be able to", Mothe points out.

False positives and false negatives

Without the possibility of being able to tell who has been vaccinated with a PCR or antigen test, what these tests do offer is the possibility of knowing if someone is infected with coronavirus. Even if they have been a few days or weeks ago, but still have a small amount of genetic material from the virus left in their body. "If you've passed the covid, sometimes the PCR will still come back positive for days or weeks", Mothe explains. In the case of antigens, it could be that the viral load is so small that the presence of the virus is not detected.

In any case, the other candidates present at the debate, with the exception of Jéssica Albiach, have criticized the "irresponsibility" of the former minister, after he spent three hours without a mask in an enclosed space. Some, like the candidate of the PDECat, Àngels Chacón, demand him to take the tests now, and others like Pere Aragonès (ERC), to take them before the next debate.

Moncunill remarks that if you want to know who has the disease in acute phase -that is, who is infected- the most reliable is are PCRs, but, even so, this will only give you a snapshot of the moment -it tells you if at that moment you are already contagious- and could change in a few hours -you could be infected but not transmit. If candidates took the test and tested negative, it is virtually certain that the result was reliable, but the same test would not be reliable the next day or two days later. Most of the candidates took an antigen test, and the PCRs, except for Ignacio Garriga's, were not from the same day either. This Thursday the nine have a date again, in this case on La Sexta. A new opportunity for controversy.