The most important problem is the coronavirus mutation and there is a high probability that those who have been ill from one strain of coronavirus may not have immunity from the new strain. I have read the statements of scientists that large countries, especially if they are not engaged in quarantine measures, and as you know, for example, the Brazilian president was an ardent opponent, are an ideal environment for the emergence of new versions of the virus that are potentially more dangerous to health and not susceptible to the action of vaccines, so I would not rely heavily on the number of COVID 19 patients to develop collective immunity.
Most of the countries that are suffering from the third wave are reporting new strains. Here in India, the double-mutated strain (B.1.617) is causing a lot of deaths, while in Brazil the Amazon variant (P.1) is responsible for most of the deaths. And this is one of the reasons why creating vaccines against viruses is difficult. The mutate so fast and the vaccine which works against the original strain may not work against the mutated strain. And this is also one of the reasons why the experts have called for equitable distribution of the vaccines. If the virus mutates, then even vaccinated population in the richer countries may not be safe.
