The concept of herd immunity (or herd, from the English term herd immunity ) is used to refer to different things. In general, it is used to refer to the indirect protection against infection obtained by susceptible people in a population when there is a high proportion of non-susceptible people (immune to infection, either after recovery from natural infection or after vaccination) in that population. .
Historical experience shows that in many infections (smallpox, polio, measles, diphtheria, pertussis, mumps, etc.) those immunized are resistant to new infections by the same agent and, if their proportion in a population is high enough, they form a barrier. that interrupts the chains of transmission.
In this way, they indirectly protect non-immunized people and, occasionally, allow the elimination or even the eradication of the infection.
Lately, and in relation to vaccination against COVID-19, the term herd immunity is increasingly used to refer to a somewhat different concept: the herd immunity threshold (HIT ). .
This threshold refers to the minimum proportion of immune persons in a population that is necessary to achieve a decrease in the incidence of infection, approximating the minimum proportion of persons in a population that should be vaccinated to protect the entire population.
In this sense, at the end of last year, Anthony Fauci, chief medical adviser to the Presidency of the United States and one of the most prestigious immunologists in the world, declared to CNN that it would be necessary to vaccinate between 70 and 85% of the population. of the United States to achieve “herd immunity” against SARS-CoV-2.
Fauci clarified that it was necessary to be “humble” with these calculations and that, “we really do not know what the real figure is”. But his prudence did not prevent the – also collective – “fever” of 70% from unleashing in the media (and governments) of many countries.
Nor did it avoid the subsequent disappointment when countries with high vaccination rates (especially Israel, see figure 1) experienced significant spikes in transmission this summer despite approaching that figure.
How is the herd immunity threshold calculated?
In its simplest form, the herd immunity threshold essentially depends on the basic reproduction number (R0). This number represents the secondary cases that, on average, an infected individual generates when the entire population is susceptible (that is, at time zero, at the beginning of the epidemic).
The very intuitive idea is that if an infected person infects only one other person (R0 = 1), the transmission will remain stable, while if it infects more than one person (R0> 1) the transmission will grow. On the contrary, if it infects less than one person (R0
From that idea, the herd immunity threshold (HIT) can be calculated as a function of R0: HIT = (1 – 1 / R0). This threshold would approximate the critical proportion of people to be vaccinated (assuming 100% vaccine effectiveness) to achieve herd immunity.
Each infectious agent has its own particular R0. It is usually higher in infections transmitted by aerosols than in those transmitted by respiratory droplets or direct contact.
In the case of SARS-CoV-2, before the expansion of the delta variant and even with much heterogeneity, an R0 was estimated at around 3.3 secondary cases per case. This data allowed the calculation of a HIT of 0.70 (HIT = (1 – 1 / 3.3) = 0.70). From there the famous 70% vaccination is extracted to achieve herd immunity.
With the delta variant, the R0 has grown. Pretty. Actually, a lot. Some estimates place it between 6 and 8. With these data, the critical proportion of people to be vaccinated would approximate percentages close to 90%.
These figures are impossible to reach when, in the first place, an important part of the population, those under 12 years of age, are not – at least for the moment – candidates for vaccination. And secondly, another part shows some reluctance to get vaccinated.
Vaccine effectiveness against infection, a missing link
The previous calculations assume the (false) premise of a perfect vaccine effectiveness to protect against infection. The term infection here refers to any type of infection (asymptomatic, mild or severe symptomatic, with or without hospitalization).
That is, they were counting on the vaccine to protect against infection. However, current vaccines were designed to be effective against symptomatic or severe COVID-19 rather than infection.
If we redo the formula to include this component, we would have: HIT = (1 – 1 / R0) / E, where ‘E’ is the vaccine effectiveness against infection. In initial studies in Israel (without circulation of the delta variant) it was estimated that the effectiveness of vaccination with a complete schedule against infection was around 80% and, therefore, HIT = (1 – 1/3, 3) / 0.80 = 0.87. 87% difficult to reach.
But in addition, this formula already indicates that if the vaccine effectiveness is less than (1 – 1 / R0) , collective immunity cannot be achieved even by vaccinating 100% of the population. And, unfortunately, for the delta variant, vaccine effectiveness against infection has been estimated between 35% and 80% and with a potential decline in effectiveness over time.
With these figures, even the most favorable (unlikely) estimates of effectiveness would not be higher than the (1 – 1 / R0) close to 90% that are estimated with delta. Consequently, in the presence of delta, herd immunity would be unattainable with current vaccines and vaccination guidelines.
In addition, there are other important factors that, even to a lesser extent, will complicate reaching the threshold of herd immunity. For example, the probability of being infected is not homogeneous between the different groups of the same population (in reality, R0 is not a function of the average number of secondary cases generated by an average infected person, but depends on how they interact with each other and with others different social groups).
On the other hand, vaccination does not occur randomly on an average population and the impact on transmission may vary depending on whether the vaccination strategy used is oriented towards populations with greater or lesser transmission capacity.
All of these are complex aspects that readers can expand on in some classic works on herd immunity, such as Fox et al. From 1971 or Paul Fine from 1993 and 2011.
When herd immunity is neither there nor expected
Having the help of “HIT = 70%” would have been good for us. But even so, and still without reaching that figure, vaccination has already allowed the wave of this summer to have been very different from the previous ones. In addition, we still have many strategies for the functional control of the pandemic. And the most immediate and important is still to vaccinate.
Vaccinate the highest possible percentage of the population and as soon as possible. All vaccines authorized in Europe have shown extraordinary effectiveness against severe coronavirus, hospitalization and death. And, so far and despite delta, this protection is maintained.
It is fundamentally individual protection, not so much collective. Only vaccinated people will have it. But it is more important now than ever because, due to vaccination’s own reduction in severe cases, restrictions will be lowered.
With this, the unvaccinated will have less and less the generic protection that non-pharmacological measures have been offering them until now (restriction of contacts, interior control, masks, ventilation).
It is also important to try to improve vaccine effectiveness. The ‘E’ in the formula. Second-generation vaccines or vaccines more adapted to the new variants could help.
An additional dose of the vaccines that we have been using could also help. But for the moment, we need more scientific evidence on these strategies.
Especially about third doses that should demonstrate not only that they increase neutralizing antibodies but how much transmission they reduce (if they do) and how much severe COVID-19 they reduce in the real world compared to previous guidelines. They are aspects to continue investigating. Even to logistically prepare your potential need. But you should also avoid overreacting without enough information.
Finally, a last important strategy is not to rely excessively on vaccination rates (on herd immunity) to stop transmission and to be guided by the rates themselves to implement or not restrictions. Mainly for those of hospitalization, but without losing sight of those of transmission.
If transmission increases there will be more serious cases. Especially in unvaccinated people, but also in vaccinated people. Even with the very high effectiveness of vaccines against severe covid, many infected people would end up resulting in hospitalizations, ICUs and deaths.
Salvador Peiró, Researcher, Health Services Research Area, FISABIO SALUD PÚBLICA, Fisabio
This article was originally published on The Conversation. Read the original.
