We will have to wait and see how and when this measure is implemented. It makes sense in the most vulnerable population, but it remains to be seen whether it will be of benefit to the rest of the population. At the moment it does not seem that the usefulness of a fourth dose in the whole population will be very high, given that people are already protected from severe disease, as shown by the current constant numbers of hospitalisations and ICU admissions.

Another aspect that will have to be considered and implemented is that, from Christmas until now, the circulation of the virus is very high and there are many infections (many of them in people already vaccinated) that involve mild symptoms, but that make these people acquire the so-called hybrid immunity, more complete than the one they had before infection, so it would not make much sense to give them a fourth dose in the autumn-winter. The problem is that these individuals, in many cases diagnosed with antigen self-testing, are not being counted and this information does not appear in the medical records of these people, which could hinder the vaccination strategy.

I do not know what evidence there may be to support this decision, as recently the WHO only recommended the fourth dose for vulnerable and older people. 

The immune response in healthy people is correct in published studies, especially in those with hybrid immunity, who have passed the infection and were vaccinated or vice versa, so an additional dose does not seem necessary.

In vulnerable people, with pathologies, immunodeficiencies, etc., a study could be made of their immune memory. This would allow a better definition of the groups and the need or not for a new dose.

Vaccine combination studies could be carried out. For example, Novavax and Hipra, which are recombinant. The vaccine against omicron seems to induce a response only for omicron and would not be useful for the other variants.

If the same vaccine is considered for further use, it does not seem to make much sense for the general population. Studies in Israel with a fourth dose show positive results, but only in the short term. After a few months the antibodies drop again.

In the case of vaccination with vaccines against Omicron, studies must first show whether it induces an improvement in the immune response in people already vaccinated or infected with the first variants. Some recent studies suggest that this may not be the case.

There is no point in continuing to administer reminder doses indiscriminately, I think it might even be counterproductive for some individuals. It is a mistake to try to maintain high levels of antibodies perpetually. Even more so when Omicron has shown partial escape of the antibodies generated by the vaccine, therefore a higher level of antibodies is needed to achieve the same neutralising action.

We must assume that with this vaccine against Omicron we cannot avoid infection in a sustained manner over time due to the natural loss of antibodies (especially in the respiratory mucosa, which are the key to avoid infection). This is not a problem for the vast majority of immunocompetent individuals who are able to cope with the virus without any problem, with conserved cellular immunity against all variants (as they do against all other viruses).

In the case of vulnerable people, I am more in favour of updating the vaccine to the omicron variant as soon as possible and starting a new primary vaccination schedule with the vaccine adapted to this variant, rather than a booster strategy with omicron, which does not seem to give good results. As long as this vaccine is not available, I think it makes more sense to carry out an exhaustive determination of immunity (cellular and antibodies) in these individuals and to select very carefully those who would benefit from additional doses. We must move away from indiscriminate vaccination by age groups and individualise the covid-19 vaccination strategy as soon as possible.