The following is my opinion, as a Roman Catholic author of several books on the End Times, including a book specifically on Garabandal and Medjugorje.
Is Covid-19 related to the Warning of Garabandal?
Covid-19 is the official name of the disease that people call the Coronavirus. This disease pandemic is not “the Warning”. The Warning of Garabandal was described by the visionaries as God showing everyone the sins on their conscience. Some have suggested that the pandemic was a way to keep everyone in their homes, to facilitate the Warning — but that did not happen. The Warning will almost certainly occur on Good Friday some year. (In my eschatology books, I speculate it will be the year 2022.)
What might the relationship be between Covid-19 and the Warning? I don’t see any direct relationship. I don’t believe the pandemic will still be affecting the world by Good Friday of 2022. There does not seem to be a direct relationship between those two events, the pandemic and the Warning.
Is Covid-19 related to the End Times?
Yes, it is related. However, in my opinion, the End Times have not yet begun. What we have been experiencing, since the year 2000, has been the threshold of the Tribulation (the Apocalypse; the End Times) — a set of preparatory events. The pandemic is one such event. It has caused people to be prepared for future pandemics, which will occur during the End Times and which will be much worse. It has caused people to store up food and supplies, which will help them during the much more severe events of the End Times. And it is itself a punishment from God for sin. If you are a devout Catholic, you know the sins of the world and how bad things have become, from a spiritual point of view. The pandemic is a punishment from God.
However, people are not punished each to the extent required by their sins (as occurs in Purgatory and in Hell). There are many grave sins in the world, generally, and so God permits great suffering, generally. Sometimes the worst sinners suffer less and the faithful suffer more.
If God were to cause everyone to suffer, just as much and only as much as they deserve, in a world of 7.6 billion persons, He would have to become very controlling. He would have to control pretty much your whole day, every day of your life. Is that what you want? God allows suffering so that He can allow us to live our lives as we choose — which implies the possibility of suffering.
But I also assure you that the world suffers far less than it would if God stepped back and did nothing. God has intervened innumerable times, in hidden ways, to lessen the sufferings of the world. His Providence will make certain that the harm of the pandemic is limited. But the best way to limit the harm of the pandemic is to cooperate with the grace of God, repent from sin, do good to those who are suffering, pray and practice self-denial.