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Immunity to the SARS-CoV-2 virus ensures protection against reinfection by this virus thanks to the combined action of neutralizing antibodies and T lymphocytes specific to viral proteins, in particular the Spike protein. It must be distinguished from the immune response that ensures healing of the infection following contamination that involves innate immunity, particularly type 1 interferons, and which is followed by adaptive cellular and humoral immunity. The importance of the effect of interferons is highlighted by the occurrence of severe forms of the disease in genetically deficient subjects or in patients with antibodies neutralizing type 1 interferon. Herd immunity is not an individual biological property. It is a mathematical property that qualifies the fact that when the proportion of subjects with individual immunity is high enough, there is little chance that an epidemic can occur. The level of that proportion—the herd immunity of the population can be computed under theoretical, often unrealistic, hypotheses, and is difficult to assess in natural conditions.

The knowledge of the global distribution of death, diseases and risk factors is important to make clear to the general public and to governments that health inequalities are incredibly high, at the dawn of this 21(st) century, and to help fight these. More than 20% of the 56 millions of deaths in 2001 were of children less than 5 years old. There are at least 1 million deaths per year from malaria. Diarrhoea kill more than 1.5 million, and measles more than half a million. The large majority of deaths by infectious diseases occur in underdeveloped countries. Moreover, chronic diseases kill an increasing number in underdeveloped countries, because populations are aging, because expansive health care which is needed to prevent and control these diseases is unavailable, and because the inhabitants are increasingly exposed to risk factors. In particular, smoking is increasing dramatically in underdeveloped countries as a result of the aggressive marketing of tobacco companies, the delay in implementing antismoking regulations, and because the public perception of the risk of smoking is still low. More than 4 million deaths per year are presently attributed to smoking, and reports forecast a death toll of 10 million in 2030. The WHO, Harvard University and the World Bank are at the origin of comprehensive data analyses on the "global burden of diseases" which help to identify health priorities. Unfortunately, global data are still scarce and of low quality, particularly in those underdeveloped countries where they would be most useful. Precise knowledge of the variations of mortality, morbidity and exposure to risk factors would be essential to monitor the improvements, or failures of health care progress. The optimal interpretation of the available data requires expertise in demography, epidemiology, statistics, and computer sciences, which are rarely found in this area. Thus, improvements in the collection of data and in the research effort in this field are necessary.

Epidemiology and heat waves: analysis of the 2003 episode in France. The heat wave that struck France in 2003 has been accompanied with an estimated 15,000 excess deaths. This paper stresses the difficulties of the epidemiology of such an event. The relevant clinical and biological information is incomplete or even inaccessible and many of the deaths are due to multiple factors. The data presently available indicate that the deaths occurred in persons already vulnerable, and that the heat wave caused a five- to eight-month loss of lifetime for the affected individuals. There is a noteworthy similarity between the profiles of this exceptional summer mortality surge, and those of many past winters when similar or larger excess mortalities ave occurred without as yet eliciting much public attention.