The goal of vaccination against covid is the same as the policy of giving the disease free rein, namely herd immunity. In this case however, the herd immunity threshold is to be reached by vaccinating people rather than reaching the goal by people actually having had the disease. But for both pathways to herd immunity, the assumption is that a person exposed to the disease, either by vaccination or natural infection, is ‘sterilised’, i.e. be in a state where they cannot (get or) pass on the virus. This seems not to be the case for everyone who has been vaccinated, as far as the data we have shows, because vaccinated individuals can still catch covid. It is clear, however, that incidences of severe disease and mortality are very much reduced among those who are vaccinated. Individuals with mild cases produce, one assumes, many times fewer virus particles than in a normal infection, and since the disease is passed on by transmitting virus particles, then vaccinated individuals will be much less likely to give others covid. It is difficult to get information about just how infectious vaccinated individuals are, because there is no way to isolate groups that contain initially only some vaccinated individuals with covid and non-vaccinated individuals that do not have covid, and see how the disease progresses.
On this basis we can distinguish three measures of vaccine effectiveness, although we might not be able to determine their values accurately. Namely, effectiveness against catching the disease Ei, effectiveness against transmitting it Et, and effectiveness against serious illness and death Es. The relationship between these measures will be Es ≥ Et ≥ Ei. Clearly, if one cannot get the disease because vaccination is effective, Ei, then the disease cannot be passed on and therefore one cannot get seriously ill. It is possible that only those who are not infected cannot pass on this disease, Et = Ei, but is seems likely that some of those who are (very mildly) infected will not pass on the infection, and hence Et > Ei. Again, it is possible that only those who cannot pass on the infection do not get seriously ill, Et = Es, the reason being that load viral loads are such as not to cause serious disease and not be sufficient for transmission, but again it is more likely that some who get mild cases can pass on the disease and hence Es > Et. One might ask if this is not all theory (and speculation), if one cannot determine the values of these measures and if that is not possible, does this not cast doubt on the overall efficacy of vaccination?
The effectiveness of a vaccine is the proportion of members of a population that the vaccine protects – we have seen that the covid vaccines can offer three kinds of protection. How is effectiveness determined? Two numbers are needed: the number of people that would not be protected if unvaccinated X and the number protected who are vaccinated Y. The ratio of these values then gives a value for E as a percentage, thus
E = (X – Y)/X × 100
Suppose 10 in a hundred people contact the disease in a given population if unvaccinated but only 1 contracts the disease if vaccinated. This gives us a value for Ei
Ei = (10 – 1)/10 × 100 = 90%
Vaccine effectiveness is a measure of how well a vaccine works. There is another measure called vaccine efficacy which refers to the results of clinical trials. Efficacy measures are reliable but restricted in scope, and unless the control group is representative of the target population, the inference to effectiveness will not be reliable. There are several different vaccines, some of which such as the Pfizer and Moderna mRNA vaccines, are somewhat better than those that use the adenovirus vector, and hence values for effectiveness and efficacy refer to specific vaccines. Moreover, as the virus mutates, it is to be expected that these numbers will change.