Immunity and herd immunity
The human body responds to infections of all kinds by producing antibodies. First, IgM antibodies are produced during a short period at the beginning of an illness, then IgG antibodies, which can last for the rest of a person’s life. These permanent IgG antibodies ensure that the individual cannot be re-infected by the same infectious agent. This is what we call “immunity.” An individual can also be immunized against an infectious agent a second way: by being vaccinated against this same infectious agent. This is the underlying principle of vaccination programs for children (diphtheria, pertussis, tetanus, hepatitis B, etc.) and adults (hepatitis, shingles, HPV, travellers’ diseases, etc.).
With respect to SARS-CoV-2, which is responsible for the COVID-19 pandemic, the scientific community still knows very little about immunity to COVID-19 and there is currently no test to ensure that the presence of antibodies confers protective immunity. Everyone is awaiting the availability of the first vaccine capable of conferring immunity, the duration of which remains to be determined.
An epidemic – or pandemic – is significantly curtailed when a large proportion of individuals have developed antibodies against the infectious agent. The infectious agent no longer easily finds new subjects that will allow it to multiply. This group or herd immunity occurs when the proportion of immunized individuals reaches more than 50%, or sometimes as high as 70%, of the population. A national vaccination program is obviously a lower-risk solution for reaching a desired percentage of immunized patients in order to achieve herd immunity.