Accumulated evidence suggests that the fourth dose of covid 19 messenger RNA vaccines, which in Spain are those of Moderna and Pfizer, will be above all essential to give greater protection to people with the most vulnerable immune systems and older people. age.
The studies published in December show, however, that a fourth dose is not capable of increasing or lengthening the immune response that had already been generated with the third dose.
What is possible is that there are vaccination regimens with recombinant proteins that can increase this response.
This is something that we must confirm with clinical studies that are already underway because, until now, almost all the studies that have been done have been with messenger RNA vaccines.
An example of a recombinant protein vaccine is the Spanish one from Hipra, which is now in a clinical study and is expected to be approved in the coming months.
At this time, and when we talk about messenger RNA vaccines, what we know is that compared to the second dose, the third increases the immune response for a longer time.
However, the fourth dose does not exceed the maximum levels obtained with the third and its effect wears off about three months after vaccination.
Therefore, the time to get vaccinated should coincide with situations in which community transmission is high and the risk of contagion is very high.
In this sense, the next booster doses of the vaccine should be planned to coincide with times of the year with greater risk, as occurs with the flu vaccine.
But we cannot forget that the fourth dose does have value for vulnerable populations.
Compared with the third, the fourth doses of mRNA vaccines administered during the period where the omicron variant has predominated reduced the risk of death for people living in residences and older people for at least two months.
These results suggest that a fourth dose can prevent mortality in fragile populations, but they also show that the protective effect is limited in time, which will force us to look for the epidemiological moments of greatest risk to vaccinate the most vulnerable.
The anticipation and ability to provide booster doses of the covid vaccine to vulnerable populations of older age or with weakened immune systems will be key to containing the excess mortality that we have continued to register during 2022 in countries where the population has a extensive vaccination coverage, such as Spain.
We must remember that, after having been infected by the virus, our immune system will maintain optimal levels to respond to a new infection for a few months and, therefore, we must take these episodes into account when deciding when to vaccinate again. since it would not make sense to receive a booster dose right after an episode of infection.
Nuria Izquierdo-Useros
is a doctor in biology, head of the emerging pathogens group at IrsiCaixa.
Question sent via email by Julio Alonso
Coordination and writing:
Victoria Toro
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