To understand the goal of a vaccination programme, one must understand the concept of herd immunity. Herd immunity occurs when a significant proportion of the population is no longer susceptible to infection. This protects the remaining susceptible individuals and stops further outbreaks. Practically, a herd can obtain immunity either by widespread natural infection or by vaccination. Some have touted the natural route to herd immunity. However, considering disease-related morbidity and mortality of SARS-CoV2, this approach is not only costly but also not ethically sound. Vaccination remains as the great hope.
The question then arises as to what percentage of the population needs to be vaccinated to attain herd immunity. To answer this, we look at the basic reproduction number (Ro) of an infection, which is a measure of the infectivity of a pathogen. It is the average number of secondary cases that result from one infected person. The more infectious a virus is, the higher this number. If Ro is above 1, then there is an assumption of a pandemic-like behaviour as one person would be infecting more than one other person resulting in exponential growth. The Ro of SARS-CoV2 was initially estimated between 2.0 and 3.1, meaning each infected person will infect on average 2–3.1 other people.(1) Measles has an Ro of 12–18, making it one of the most infectious viral illnesses. Once the Ro is known, the herd immunity can be calculated using the formula 1 − (1/Ro).(2) From this formula, the more infectious a virus is, the higher the number people need to be immune before achieving herd immunity.
Although Ro is elegant in its simplicity, the problem lies in the assumption that the whole population is fully susceptible, which is generally incorrect. As a pandemic progresses, people become infected and therefore become immune, and others will increasingly apply infection prevention measures. A drop in infectivity and therefore a decline in Ro will ensue. This makes Ro a moving target, which would mean that the numbers to calculate herd immunity would continuously change. The difficulty in calculating Ro is further illustrated by a letter published in March 2020. Singapore was noted to have an Ro of 1.13 and therefore required 11.5% of the population to be immune, whereas Spain had an Ro of 5.17, which required 80.7% of the people to be immune.(3) Clearly, this does not reflect differing viral behaviour but rather the impact of differing infection prevention measures. Further, central to the concept of herd immunity is immunity. The assumption is that once infected or vaccinated, a person would remain immune. This is generally the case with infections such as measles. However, with SARS-CoV2, reinfection is now described, either by waning immunity or by variant-related disease. Ro is thus somewhat of a mythical construct, and the concept of herd immunity it subtends may at best be described as a mirage.
THE BEST LAID PLANS OF MICE AND MEN
Logic dictates that if more people are vaccinated, the outlook would be more favourable. In opposition to this goal is the vaccine denier or the somewhat newer vaccine hesitater. In their guide entitled “How to respond to vocal vaccine deniers in public,” the World Health Organisation has condensed a practical approach to dealing with vaccine deniers. A denier is described as a person who ignores evidence and adheres to a belief that is impossible to challenge. It is accepted that in a situation analogous to religious and political fundamentalists, deniers are a lost cause in terms of conversion, and efforts should be aimed at containing and limiting their toxicity from spilling over to the hesitators. Unfortunately, past vaccine misadventures have created fertile ground for scepticism. In 1955, the Cutter company did not fully inactivate the polio vaccine, resulting in an iatrogenic polio epidemic. In 1976 the swine flu vaccine may have led to an increase in Guillain–Barre cases. As recent as 2017, dengue vaccinations resulted in more severe infections in seronegative subjects (although to be fair, this group was explicitly excluded by the manufacturer). The greatest blunder was probably an outbreak of hepatitis B in 1942 where US Army recruits were vaccinated with a contaminated yellow fever vaccine. The outbreak infected over 300,000 troops and resulted in 50,000 clinical cases of hepatitis.(4)
Although deniers are easy to dismiss as a fringe group, vaccine hesitators are much more substantial in number. A poll found that 35% of Americans would not accept a free US Food and Drug Administration (FDA) approved vaccine.(5) In South Africa, a similar picture has emerged. A survey found that 18% of respondents said they would not take the vaccine, and 15% were unsure if they would take the vaccine. Reasons for non-acceptance included (6)
Concern regarding effectiveness related to the validity of trials or speed of vaccine development (18%)
Vaccine distrust (14%)
Conspiracy theories (7%)
Felt that they were not at risk of infection (5%)
Distrust of institutions (6%)
Concerns related to the occult (4%)
There is no doubt that social media fuels hesitancy. In an analysis of the dengue saga mentioned earlier, several types of misinformation were identified and included, unsubstantiated death tolls, online conspiracies, rumours implicating pharma and health officials, health hoaxes, fake cures and scams.(7)
Similar tactics are currently at play with COVID-19. The FDA is sensitised to the impact that hesitancy can have on herd immunity. In a viewpoint published in Journal of American Medical association, the following safeguards were proposed:(4,8)
THE GREAT VACILLATION: OXFORD–ASTRAZENECA SAGA
A consignment of the Oxford–AstraZeneca vaccine arrived in South Africa with some pomp and ceremony. There has now been a change of minds. This vaccine was never a chart-topper in terms of efficacy, especially in older patients. The Swissmedic report stated: “The data currently available do not point to a positive decision regarding benefits and risks”.(9) Although one does not wish to diminish the complexity of decisions regarding vaccine selection, the current vacillation does feed into the reasons for non-acceptance mentioned previously. The Johnson & Johnson offering, also an inactivated adenovirus, is favoured, but there has been a subtle change in language. No longer is there talk of herd immunity and liberation from masks and social distancing. Now the selling point is the prevention of severe infection. If intentional, this is telling of a more realistic appreciation of the SARS-CoV2 pandemic. Prevention is a sliding rule with 100% vaccination on the one end and 100% isolation/infection prevention on the other. The extremes are unrealistic. The much sought-after herd immunity is likely not obtainable and if fewer people accept the vaccine, the slide rule can compensate by shifting to the side of infection prevention. We fret about the deniers and the hesitators as we seek liberation, but the only liberation that is currently on offer is that of accepting the status quo.