the third dose hypothesis and the months of protection –

the third dose hypothesis and the months of protection –
the third dose hypothesis and the months of protection –

How long will the protection given by vaccines last?
We don’t know yet, because there is no test that can predict the duration of protection. And this applies to all vaccines in use. Immune responses and the level of protection induced by vaccines are being followed throughout the world over time: the prevailing hypothesis that boosters will be necessary, but it is not clear how and when they will be carried out. By analogy with the vast majority of vaccines, it is reasonable to assume that protection will last at least a year.

How many months of immunological memory have been monitored so far?
Phase 3 studies started 8-9 months ago and tell us that vaccinated people are still protected. In the UK and Israel, where vaccinations started in December, the protection given by the first vaccinations still applies.

Why is this still uncertain?
SARS-CoV-2 is a relatively new virus and does not help the comparison with other coronaviruses, which have been poorly studied because they cause mild symptoms, nor with SARS, which disappeared within months. We do not even know what the correlation of protection is, a measurable numerical level of immune response above which we are protected and below which we are susceptible again. The other confounding factor in the possible re-exposure by vaccinated people to a virus that still circulates: exposure refreshes the immune memory and raises antibody levels. Monitoring should be able to exclude this factor in order to be able to determine the duration of vaccination protection in itself.

How is the duration of protection monitored?
Not having a correlated protection, we are based on empirical data deriving from the continuous measurement of the immune response over time (antibody and T lymphocytes) and the number of reinfections in the already fully vaccinated population. We know that the immune response decreases over time, but, as written above, we do not know what is the point below which we are no longer protected. It may well be that even a halved or 5-10 fold lower immune response level still protects against reinfection. It will also be necessary to establish whether the third dose (the second, in the case of single-dose vaccines) will be a booster against a homologous or heterologous variant of the virus.

What does it mean? What kind of vaccine will the third dose be made with?
Vaccine boosters can be of two types: homologous, that is, against the same micro-organism (as with tetanus, which is done every 10 years), or heterologous, as in the case of influenza for which the vaccine must be reformulated every year to counteract a different viral strain. The homologous recall is what will be done when the immune response should weaken to the point of exposing itself to re-infection against the same variants that circulate today. The heterologous recall is what would be done against new possible variants of SARS-CoV-2 that were no longer effectively neutralized by the current vaccines.

Will emerging variants change our vaccination needs regarding boosters?
Surely the vaccination policy will have to follow the evolution of the viral variants.

If the variants were to reduce the effectiveness of vaccines, would the months of protection guaranteed by them also be reduced?
It is possible that protection from less effective Covid-19 vaccines will wear off more quickly, but we still don’t know.

Are there any differences between vaccines regarding boosters? Can some apply for them earlier than others?
We will find out over the months: we must take into account that many vaccines in use are products on which there is no experience: messenger RNA drugs (such as Pfizer and Moderna) did not exist before, viral vector vaccines (such as AstraZeneca and Johnson & Johnson) existed, but had never been used extensively to vaccinate entire populations. We do not know if one type of vaccine over another will provide a longer lasting memory.

Qwhat vaccine combinations will there be for the third dose? Same? Or specific to the variants?
It depends on the regulatory agencies and the data we will have available.

* Sergio Abrignani, immunologist of the University of Milan collaborated.

June 8, 2021 (change June 8, 2021 | 09:13)



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