Why do vaccinated people test positive for COVID-19?

Many people who have been vaccinated against COVID-19 believe that they won't catch COVID-19 since they got the shots.

Let's see how realistic that belief is.

Thousands of so-called "breakthrough cases" – a COVID-19 infection despite full immunity after vaccination – around the world have proven that some get infected despite being vaccinated, especially in nursing homes.

This comes in spite of the fact that major vaccines approved in Western countries offer a high level of protection, even if this varies from vaccine to vaccine.

The mRNA vaccines from BioNTech/Pfizer and Moderna reduce the risk of contracting COVID-19 by up to 95% against original COVID-19 variants.

The effectiveness of the so-called vector vaccines is somewhat lower after full immunization: up to 80% for the vaccine from AstraZeneca, while Johnson & Johnson calculated effectiveness of 66% for its vaccine.

But even if the vaccines significantly reduce the risk of contracting COVID-19 and their benefits vastly outweigh any risks, there is no 100%t protection.

As with other viruses, so-called vaccine breakthroughs can occur. The probability that someone will become infected with the COVID-19 and develop symptoms despite being fully vaccinated is "low, but not zero," according to Germany's Robert Koch Institute (RKI), the health body helping coordinate the country's pandemic response.

Vaccination is also not a guarantee against a serious case of COVID-19, and among 5,374 such "breakthrough cases" in Germany since the beginning of February, 676 affected persons had to be treated in hospital, 614 of whom were older than 60.

We can also assume that there is a high number of unreported cases of vaccination breakthroughs.

Why COVID-19 outbreaks are still occurring in nursing homes in particular, despite complete vaccination of the residents, was recently investigated by the Charite hospital in Berlin.

According to the researchers, the fact that vaccines usually work more efficiently in younger people is mainly due to our immune response decreasing with age.

Deficits in the immune response are sometimes also found in younger people – for example, when the patient's own immune system is specifically suppressed with medication after an organ transplant.

Data showed "that the immune response can be much worse in organ transplant patients depending on the immunosuppression," said the chairperson of Germany's Permanent Vaccination Commission, Thomas Mertens. "It is then only 50%"

The immune response can also be weaker in rheumatism and cancer patients.

In addition, some COVID-19 variants, such as the delta variant (B.1.617.2) first discovered in India, can slightly reduce the efficiency of vaccines.

However, current research suggests that the vaccines also protect against such variants. The vaccines from BioNTech/Pfizer and AstraZeneca, for example, are hardly less effective against delta than against the alpha variant originally discovered in Great Britain, U.S. researchers recently reported in the New England Journal of Medicine.

The prerequisite, however, is complete immunization. After only one vaccination dose, the effectiveness is considerably lower.