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Antiviral jabs: Can vaccinated people still be contagious?

Antiviral jabs : Can vaccinated people still be contagious?

Vaccination dramatically reduces the risk of contracting life-threatening covid-19. But does vaccination also protect others from infection? It's not quite that simple, experts explain.

The holy grail of a medical study is the so-called endpoint. It is the crux to which all statistics are directed. In the trials for a vaccine against coronavirus, the endpoint was: Can it reduce the number of severe cases? That question was answered very convincingly in the affirmative in the Phase III trials. However, the results would have been even more powerful if all subjects had also been regularly tested by PCR to determine whether they could become infected despite vaccination; this could also have been tested as a secondary endpoint. After all, viruses can live on in mucous membranes without penetrating the cells and causing damage. This is called clinical immunity. Nevertheless, it would be possible for them to be passed on to others - perhaps in smaller quantities, but still.

Potentially infectious

Düsseldorf virology professor Jörg Timm is familiar with the data situation: "In the approval studies for the Biontech and Moderna vaccines, infections were observed significantly less frequently in the group of vaccinated patients than in the control group vaccinated with placebo. From the available data, however, it is not yet possible to conclude with certainty whether the vaccine actually prevents asymptomatic infections as well - i.e., confers so-called sterile immunity - or whether it primarily prevents symptomatic and severe cases, which are usually the reason for testing." For Klaus Cichutek, head of the Paul Ehrlich Institute, there are two sides to the issue: "There is no good evidence from animal studies that we can really achieve sterile immunity." Nevertheless, he believes "that if there is a reduction in severe cases, at least a reduction in viral load in the upper respiratory tract happens after all.“

And what does the animal research say? Well, it's known from studies on vaccinated monkeys that while they don't get sick when infected with Sars-CoV-2, they do have active viruses in their noses that they can also spread. On the other hand, they had been sprayed with such a high amount of viruses in their noses, which clearly surpasses a normal human infection in terms of viral load. Nevertheless, the results of the animal experiments are an indication that vaccinated individuals are considered potentially infectious for the time being and therefore cannot be exempted from a mask requirement.

How long does immunity last?

Wolf-Dieter Ludwig, chairman of the Drug Commission of the German Medical Profession is also sceptical about immunity, and sterile immunity at that: "We know nothing about how long this immunity lasts. We know for relatively sure that so-called sterile immunity is probably not achievable at all at the moment.“

Timm, like Cichutek, is more optimistic about the question of whether vaccinated people can still pose a risk of infection: "Even if sterile immunity is possibly lacking, I assume that in the case of infection after vaccination, the amounts of virus in the respiratory tract are significantly lower. Therefore, in any case, vaccination will have a positive effect on the spread of Sars-CoV-2 as long as people do not show increased risky behaviors."

Research to date shows that the vaccines elicit a better antibody response than natural infection. To be sure, vaccination cannot be assumed to provide lifelong protection. But, even if the protection lasts only two years, it is possible to revaccinate. This would be all the easier if the vaccine could be administered by nasal spray or lozenge. Such methods are currently in the pipeline. Despite all the mutations, Sars-CoV-2 appears to show much less variability than the seasonal flu virus.

"Natural immunisation"

Nevertheless, some people wonder whether natural immunity after having been infected might not be the safer protection against re-infection. The argument in favor of this "natural immunity" option is that the immune system's reaction to the entire virus must be stronger and more reliable than the reaction to just one surface feature - in this specific case, the spike protein against which the antibodies of the new vaccines are directed. To refute this theory, it is sufficient to take the example of vaccination against hepatitis B: Here, too, a tiny feature is sufficient to achieve the desired immune response. The immune system always identifies a virus on the basis of individual criteria. For Sars-CoV-2, the characteristic spikes are decisive (which in their basic structure are also not threatened by mutations).

Importantly, the new vaccines also appear to stimulate the body's cellular immune response that would otherwise be elicited by infection. This is another arm of the defense system that is recruited from memory cells (T cells). They can remember individual characteristics of pathogens even months and years after an infection and, when they encounter them again, trigger the production of antibodies and special lymphocytes that attack and kill the viruses. The Corona vaccines trigger a similar cellular immune response.

Virologist Timm says: "It's important now to closely monitor whether new viral variants may be playing a role in post-vaccination infection cases for which the vaccine is ineffective."

Original text: Wolfram Goertz

Translation: Mareike Graepel