Why you don't need a test after vaccination to confirm immunity

This article was published on
August 6, 2021

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Vaccination protects against the most severe symptoms of covid-19 and does not require a test to confirm this.

Vaccination protects against the most severe symptoms of covid-19 and does not require a test to confirm this.


What our experts say

Context and background


Information and context on vaccines. Article written by the science journalists of the SINC agency with the analysis and review of expert sources.

The aim of serology is to detect cases that had not been identified during infection because they were mild or asymptomatic, not to determine the level of protection by the vaccine.

It takes about two weeks for a person who has received the full course of vaccination to develop maximum protection against the most severe forms of COVID-19, although he or she starts to develop defences from the first injection in the case of double-dose injections.

For example, people vaccinated with Pfizer/BioNTech reach peak immunity within seven days of the second dose, while those vaccinated with Oxford/AstraZeneca take about 28 days.

In this sense, it is not necessary to have a serological test after vaccination to confirmimmunity. “The efficacy of the vaccines is very high,” claims José Ángel Hernández Rivas, a member of the Federation of Spanish Scientific MedicalAssociations (FACME, from its Spanish initials) and of the Spanish Society of Haematology and Haemotherapy, who, in general, does not see the need to checkit, except in immunosuppressed patients with severe immune system alterations or in those who receive treatments that may compromise the serological response.

In addition, serological tests were not evaluated to determine the level of protection after vaccination, but rather to establish whether a person has previously overcome SARS-CoV-2infection by detecting antibodies in the blood.

In fact, there are even agencies, such as the US Food and Drug Administration (FDA), that advise against the use of serological tests for this purpose.

The aim of this type of test is to determine the magnitude of an epidemic or outbreak in a population, as in the case of the ENECOVID study, to detect, mainly, those unknown cases that had not been identified during infection because the disease was mild or asymptomatic, as the World Health Organisation (WHO) reminds us.

Being on the alert for false negatives

The result of some serological tests after vaccination may be negative, as some of these tests are based on antibodies that are not those that the body develops after receiving the full course of vaccination.

Most serological tests are designed to detect antibodies that are part of the immune response to infection. Thus, these tests focus on antibodies to the N protein, which are those generated by the body after a natural infection, not by vaccination.

In contrast, in the case of European-approved messenger RNA vaccines, such as Pfizer/BioNTech and Moderna, the drug induces defences against the corona spicule of the virus, the S protein, which attaches to human cells and which most serological tests do not detect.

“The best protectionis provided by vaccines,” says Hernández Rivas.


Antigen tests

Vaccination would not generate false positives in antigen tests, since “the protein production generated by the vaccine is local and it does not seem possible that a part of this protein could reach the respiratory tract in a detectable quantity,” where the nasopharyngeal sample is taken by means of a swab, according to a FACME communiqué.  

There are four different vaccine types being administered in Spain. Two of them, Pfizer/BioNTech and Moderna, are based on messenger RNA technology, and the other two, Oxford/AstraZeneca and Janssen, on the introduction of the inactivated virus. None of the four can reach the respiratory tract and therefore result in a false positive.

An antigen test positive or a PCR mean SARS-CoV-2 infection, which vaccinated people are not safe from, as vaccination protects against the more severe forms of COVID-19 and death. This is why vaccinated people should continue with protective measures.

This article is also available in Spanish.

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