Already with figures calculated using data related to the COVID-19 Delta variable, the threshold for herd immunity would be set at 90% of the population vaccinated. If the calculations are applied to the more contagious Omicron variable, the figure would reach 95%, a figure which, taking into account the population unable or unwilling to vaccinate, would be unattainable. These are the conclusions reached by the study entitled Caveats on COVID-19 herd immunity threshold: the Spain case, which has been published in the journal Scientific Reports by an interdisciplinary team of researchers from the University of Alicante departments of Applied Mathematics and Ecology and the Ramón Margalef Multidisciplinary Institute for Environmental Research (IMEM).
The research team, led by David García, lecturer in the Department of Applied Mathematics at the University of Alicante, also includes César Bordehore, from the Department of Ecology and the Ramón Margalef IMEM; Eva S. Fonfría, from the Ramón Margalef IMEM, and Isabel Vigo and Enrique Morales, also from Applied Mathematics.
According to the publication, the study covers a detailed analysis of the herd immunity threshold of the ancestral COVID-19 variant, which was dominant at the beginning of the pandemic, from different approaches. This quantifies the influence of three key factors such as the source and quality of the data, the evolution of infectivity over time and the methodology for estimating the R0, which is an essential value in these calculations because it establishes how many people, on average, would be infected by an infected person at the beginning of the pandemic — i.e. when no preventive measures were taken, no preventive use of masks was made, nor was the population warned of the existence of a contagious disease that would cause them to change their behaviour.
On the detailed analysis of the HIT (Herd Immunity Threshold), the study allows indirectly estimating a new value of R0 with the current dominant variants, such as Delta (at the time of the study) and Omicron (according to the research team's estimates after the publication of the study), which results in a significant increase in the percentage.
In the same vein, the research reveals that a very influential factor for the estimation of HIT is the methodology for estimating R0, which can produce differences of between 20 and 30 percentage points for the same data set to which a dynamic calculation model is applied. In this situation, the study takes the highest value, the so-called upper bound, for the herd immunity threshold, in order to achieve the highest fidelity in the prognostic calculations, as García explained.
In the case analysed, the infection data used by the UA researchers were estimated using a retrospective methodology called REMEDID (Retrospective Methodology to Estimate Daily Infections from Deaths). This methodology, also published by the UA researchers in a previous scientific article in the same journal (Scientific Reports), is capable of estimating daily infections from the number of deaths, a figure that helps to analyse and understand the dynamics of the pandemic, and to evaluate the efficiency of the measures adopted at different spatial and temporal scales because it allows the calculation of time series of daily infections from time series of daily deaths.
On the results of the study, researcher David García explained that herd immunity was at 70%, as commonly established by official sources from the beginning of the pandemic and when vaccination began. However, with the new variants, estimates of herd immunity would be above 95% with the Omicron variant.
This percentage of 95%, is an unattainable figure because there are people who cannot be vaccinated and people who do not want to be vaccinated. Also, they added that vaccines do not prevent the transmission of the virus, as we saw last Christmas, when, despite the high vaccination percentage, we saw the strongest wave, yet with minor consequences due to the protection provided by the vaccines. In this new scenario, a higher percentage of vaccinated people is required to achieve herd immunity, as the effectiveness of vaccines has been key to reduce mortality. That is why it is more necessary than ever that as many people as possible get vaccinated.
The case of Spain
Some of the most relevant data are extracted when the calculations of the study are placed in the special features of the pandemic in Spain. The results presented are valid for a randomly mixed population with similar spread dynamics to Spain as a whole. However, even Spanish regions show different dynamics from each other, which may lead to region-specific HIT values.
According to Garcia, it should also be borne in mind that none of the three vaccines administered in Spain is capable of completely preventing transmission of the virus, so that even with 90% of the population vaccinated, the HIT will probably not be reached. However, the risk of infection is significantly reduced for vaccinated people, which directly reduces the R0. Furthermore, in the event of infection, transmission of the virus is also reduced, which means that, although transmission is not completely prevented by vaccines, the higher the proportion of vaccinated population, the lower the herd immunity threshold, the UA researcher added.
Regarding the distribution of the pandemic by region, García commented that even if HIT is achieved, it will not be a universal remedy. If herd immunity is achieved in most parts of a country, but there are some specific regions or population subgroups in a region with a percentage of immune individuals below the HIT, local outbreaks will be possible in those regions or subgroups. In reference to the original variant of the virus, if the ancestral variant had not been replaced, the decreasing rate of infections after reaching a HIT of 70% could still produce a non-negligible 25.9% of infections, i.e., 12.2 million infections in Spain.
Finally, the researcher concluded that health authorities should strictly guarantee an adaptive and proactive management of the new situation once a theoretical herd immunity has been achieved.
Photo: The UA interdisciplinary team that published the study, with César Bordehore, Isabel Vigo, Enrique Morales and David García, from left to right
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