Covid-19, five years later: the virus is still there, even if we can't see it.

Just over five years ago, in December 2019, the first cases of an unknown pathogen were detected in Wuhan, China: SARS‑CoV‑2 . Since then, the combination of vaccination and natural infections has formed what immunologists call a " wall of immunity ," a barrier that, to a large extent, protects us from severe cases and mortality associated with the infection.

Let's imagine a large schoolyard where, in different seasons, respiratory viruses—such as influenza, RSV, and now also SARS-CoV- 2—are seen out in groups for recess. The difference is that the latter seems to have made its mark in the summer. As a prestigious virologist friend of mine always tells me: "Viruses, like firefighters, don't step on each other's hoses." Therefore, each one chooses its own time . And SARS-CoV-2, contrary to all intuition, is not slowed down by high temperatures: it continues to replicate and transmit easily even in summer, further fueled by increased social contact.

The success of vaccines is paradoxical: precisely because our immune system is protected, the virus It becomes invisible, creating a false sense that the danger has passed . But it's still there, lurking behind the wall of immunity . And that wall isn't the same for everyone: while healthy, immunocompetent people live peacefully on the other side, others see their protection weaken. Older people—due to immunosenescence—with chronic illnesses, obesity, or immunosuppression can see that wall lose bricks over time.

That's why it's essential not to let our guard down. Regular vaccination of at-risk individuals—such as the flu campaign—works as a booster, restoring protection and strengthening the barrier where it has weakened. The constant emergence of variants— omeron and subvariants —some capable of evading part of our immunological memory , reinforces the need for updatable vaccines. These adapted formulations not only rebuild the barrier but also correct the "imprint" left by the first exposure to the virus, making our immunity more effective against new strains.

In this scenario, epidemiological surveillance plays a key role . Monitoring the evolution of the virus and its circulation, not only in SARS-CoV-2 but also in other respiratory pathogens, helps anticipate possible "escapes" from the barrier. Early detection allows for the rapid administration of booster doses to vulnerable groups, minimizing the risk of serious outbreaks.

Yet the media often contributes to vaccine fatigue. Sensational headlines with catchy names—"Omicron," "Eris," "Pirola"—stir up unnecessary alarm . The result is that the public, hearing alarm after alarm, convinces itself that COVID is no longer serious and that vaccines are unnecessary—a dangerous mistake when the wall cracks in the most vulnerable.

This is where the irreplaceable role of experts and health authorities emerges. Their communication must be clear, truthful, and scientifically justified. Only in this way can clickbait be countered and a true understanding of the risk be promoted. Each recommendation must be explained with data: which group it affects, which variant is circulating, what protection the vaccine offers, and when new vaccination waves are scheduled to begin.

Fortunately, we now have new technologies, such as mRNA, that are rapidly adaptable . Their rapid reprogramming capacity opens the door to targeted vaccines against newly emerging variants. This allows for the design of campaigns each season that respond to the predominant strain, thus aligning reinforcement and protection, and preventing the immune system from becoming locked into its memory of the original virus.

If this trend of summer seasonality of SARS-CoV-2 is confirmed, it will also be necessary to rethink our vaccination strategy. The traditional fall-winter campaign may not be the best time to immunize against COVID if its peak incidence is consolidated in the warmer months. In that case, It will be necessary to assess whether separating the flu and COVID-19 campaigns can offer better results. It is up to public health experts to determine the most appropriate timing for each vaccination , taking into account epidemiology, logistics, availability, and supply of doses. The important thing is that each virus is addressed at the time it poses the greatest risk to the population, with the maximum guarantees of protection.

In short, five years after the first outbreak in December 2019, severe COVID-19 is no longer the monster that changed our lives. But the virus is still there, in our backyard, ready to return if we give it a chance . Our wall exists, but it needs maintenance. That means:

• Maintain targeted, updated, and recurring vaccination campaigns , especially if summer circulation of the virus is confirmed.
• Strengthen epidemiological surveillance to detect variants and anticipate strategies.
• Communicate honestly, guided by experts and authorities, avoiding alarmism and explaining the reasons for each dose.
• Be aware that collective protection is not eternal : it must be strengthened, especially for those who, due to age or illness, cannot build it on their own.

This wall protects us all: it's not just an individual issue, but a shared responsibility. Those who can protect themselves must do so to sustain the common bloc. And those most at risk must be guaranteed access to the vaccines that keep them safe. Only then, five years later, can we confidently say that we haven't lost the lessons learned in that first winter of 2019.