Why getting vaccinated against flu and covid today does not protect you from getting infected immediately

The vaccination campaign against flu and covid viruses is advancing against the clock in the autonomous communities in the middle of autumn, just when the rise in infections begins, which usually reaches its epidemic peak during the winter months, that is, coinciding with the Christmas holidays and until February.

These are two vaccines that are updated every year and therefore it is advisable to reapply them, since the antibodies produced by people thanks to vaccination decrease over time. In addition, flu and covid viruses tend to suffer variations in their molecular structure that allow them to escape the protection of a previous vaccination, so it is very likely that people who have previously been infected or have been vaccinated in other years are not protected against the new mutations.

However, getting vaccinated against flu or covid does not imply immediate protection against these viruses, which is why there are cases of colds shortly after being vaccinated. The Ministry of Health explains that this is due to several factors, such as the presence of other respiratory viruses, but also because immunity against flu or covid takes several weeks.

In a document published for information purposes, the Ministry first explains that during the winter months numerous respiratory viruses coexist in addition to flu and Covid, such as rhinovirus, respiratory syncytial virus (RSV) or adenovirus, which also spread and can cause illness during the flu season. The flu and covid vaccines only protect against flu and covid viruses "and not against these other viruses," they specify.

Furthermore, in the case of the flu, after receiving the vaccine "it takes two weeks for the body to develop immune protection."

This explains the possibility of "having been infected by the flu virus immediately before being vaccinated or during the two weeks after vaccination, so a person could have the flu despite having been vaccinated," they detail from Health.

Another reason for contagion despite the vaccine may be due to the person becoming infected with a flu virus other than those included in the vaccine or not being adequately protected due to advanced age or health status. "The ability of vaccines to protect a person against the flu depends, to a large extent, on the coincidence of the viruses selected to make the vaccine and those that circulate in that season, and on the ability of the immune system to generate immune protection," they clarify.

Regarding covid, Health indicates in another informative document on vaccination that "optimal protection" against this disease "is achieved approximately two weeks after administering the vaccines."

As detailed, "protection begins to develop gradually after a few days and can be significant from the first week if it is a booster dose, but it is around two weeks when the highest concentration of antibodies is reached."

Both vaccines can be administered together "in different anatomical sites", for example, one in each arm. "The available evidence shows that no difference has been observed in the immune response of the flu and Covid-19 vaccines after their joint or separate administration," indicates Health.

Although the vaccine does not 100% prevent a person from contracting the flu or covid, the disease "will most likely be less serious", so hospital admissions or major complications will be avoided in a high percentage.