By Matías A. Loewy
Herd immunity, or collective immunity, became a kind of holy grail of the SARS-CoV-2 pandemic, the light at the end of the tunnel that, thanks to vaccination, would allow us to return to normality as we knew it. However, experts say there are misconceptions that should be clarified so we don't let our guard down prematurely.
The first detailed description and discussion of herd immunity appeared in a classic 1929 book, The Principles of Bacteriology and Immunity , by Topley and Wilson. It stated that “the English herd is immune to plague and typhus, but only as long as the individuals remain within the herd,” recalls Thomas Hugh Pennington, Ph.D., emeritus professor of bacteriology at the University of Aberdeen , in Aberdeen, Scotland, in an article .
In the context of COVID-19, the concept of herd immunity is more restricted and refers to a level of specific immunity against the SARS-CoV-2 virus above which the basic reproduction number ( R0 ) falls below 1. R0 is a measure of the number of people to whom an infected person can transmit the disease, or the number of secondary cases that each primary case generates on average (during the time they are contagious). When the basic reproduction number is less than 1, the situation is heading towards what is called "epidemic exhaustion."
Mathematically, the herd immunity threshold (the point at which the proportion of susceptible individuals in a population falls below the threshold needed for transmission) is defined by the formula 1 – 1/R 0 , which implies that the more transmissible a pathogen is, the greater the proportion of people who will need to be immune to block sustained transmission.
But how far are we from reaching that point with COVID-19? And what are the implications? Here are five myths or "half-truths" about herd immunity worth considering.
1. 70% of the population needs to be immunized to achieve herd immunity.
The idea that 70% population immunity is needed to curb COVID-19 transmission stems from estimating a basic reproduction number (R0) of 3 (2.5 to 3.5) for SARS-CoV-2 and applying the formula 1 – 1/ R0 , implying that 67% of the population should be immunized. Since vaccine efficacy is not 100%, this means a higher proportion of people would need to complete the vaccination schedule. Most models place this threshold between 60% and 80%.
However, the basic reproduction number and the effective basic number (R, or the number of people who become infected when there are already immune individuals or containment measures have been implemented) can vary depending on factors such as adherence to social distancing measures, the demographic structure of the population, and the proportion of new circulating variants. Furthermore, the herd immunity threshold assumes a homogeneous population distribution, which remains a theoretical approximation.
The herd immunity threshold can vary from country to country and between regions within the same country, Dr. Mario Fontán Vela, a member of the Public Health and Epidemiology Research Group at the Faculty of Medicine and Health Sciences of the University of Alcalá, in Alcalá de Henares, Spain, explained to Medscape in Spanish.
"The pattern of socialization between population groups and interactions between different territories also influence this threshold. In the end, there are many factors to consider that can raise or lower this threshold, which is still a very theoretical calculation as a reference point... I think we shouldn't get too obsessed with it," noted Dr. Fontán, a resident physician in Preventive Medicine and Public Health at the Preventive Medicine Service of the Infanta Leonor University Hospital in Madrid, Spain.
"We will reach herd immunity when enough doses of the vaccines reach the countries, but it is difficult to predict how many people need to be vaccinated in each country to reach that point. 70%? 80%? 90%? We will know when there are practically no new infections and this remains the case," admitted Dr. Carissa F. Etienne, director of the Pan American Health Organization (PAHO), on April 14.
2. Herd immunity is achieved simultaneously with any vaccine.
Although natural immunity from infection also contributes to herd immunity, the general consensus is that it can only be achieved through vaccination . However, determining the estimated proportion of the population that should receive vaccines would require adjusting for the effectiveness of different vaccines.
Zoë Hyde, Ph.D., an epidemiologist and biostatistician at the University of Western Australia in Perth, Australia, calculated in The Conversation that for a basic reproduction number of 2.5, almost 100% of the population needs to be immunized with a vaccine that has 62% efficacy, such as the Oxford/AstraZeneca vaccine, but 63% coverage could be sufficient if one with 95% efficacy, such as the Pfizer/BioNTech vaccine, is used (since Australia achieved effective control of community transmission and has fewer than 30,000 SARS-CoV-2 infections out of approximately 25 million inhabitants, all population immunity should be achieved through vaccines).
Kamran Kadkhoda, Ph.D., an immunopathologist at the Cleveland Clinic in Cleveland, USA, estimated that even with a basic effective number as low as 0.99, herd immunity of 60% to 72% is required to break the chain of transmission, implying that 63% to 76% of the population should receive a vaccine with 95% efficacy, or 84% to 90% if a "decent" margin of safety is desired.
The goal of herd immunity depends directly on the vaccine's efficacy. For example, if the entire population is immunized with a vaccine that has 50% efficacy, it will not reach the desired 80% target to curb transmission, Juan Jaworski, Ph.D., a virologist and vaccine researcher at the National Scientific and Technical Research Council of Argentina at the National Institute of Agricultural Technology, told Medscape in Spanish .
3. After reaching the herd immunity threshold, there can be no new infections or outbreaks.
That's another common misconception. Reaching herd immunity is like trying to slow down a car traveling at 100 km/h: it will eventually stop, but until then it may run over pedestrians and crash into other vehicles. Similarly, the virus will continue to circulate for some time.
In a scenario where herd immunity has been achieved and each person positive for SARS-CoV-2 infects an average of 0.8 others, 100,000 infected people will still transmit it to 80,000, and those 80,000 to 64,000, and so on. Many hospitalizations and deaths can still be expected.
On the other hand, most models assume homogeneous populations, but in reality people mix differently and the networks of contacts with which each person interacts become important, Kit Yates, Ph.D., professor in the Department of Mathematical Sciences at the University of Bath, in Bath, England, and author of the book The Numbers of Life: Seven Mathematical Principles That Shape Our Existence (2019), told Medscape in Spanish.
“For example, young people tend to mix more with other young people. If we have a large proportion of the population without immunity, even if one of those theoretical herd immunity thresholds is exceeded, we could still see outbreaks among younger people. The paradox is resolved because models that assume well-mixed populations are not a good description of reality, and it could be argued that talking about a single threshold for herd immunity across the entire population is not helpful,” he noted.
Dr. Yates clarified that he does not mean to imply that herd immunity is not important to curb the spread of COVID-19, but rather that it is difficult to define a single critical threshold beyond which the effective basic number drops below 1.
4. Manaus exemplifies how new variants violate herd immunity.
Manaus, the capital of Amazonas state in Brazil's Northern Region, became a cautionary tale. After enduring a precipitous peak in infections between April and May 2020, COVID-19 cases and hospitalizations remained very low until November. A study of blood donor samples seemed to explain this, estimating that by October , 76% of the city's inhabitants had been infected —an attack rate exceeding the theoretical herd immunity threshold of 67%.
However, in December 2021, the Amazonian city experienced an unexpected resurgence of cases driven by the emergence of the concerning P.1 variant. "The current situation in Manaus indicates that herd immunity is not lasting or that this new lineage manages to escape the protection provided by previous antibodies," declared Dr. Ester Sabino, a physician and professor at the University of São Paulo , in São Paulo, Brazil, that month.
Since then, Manaus has become a recurring example of how the emergence of a new variant could quickly limit herd immunity. However, there may be an alternative explanation: Manaus never actually reached herd immunity, and cases declined due to the adoption of public health measures, suggests a study published as a preprint on medRxiv.
According to researchers led by biomathematician Lewi Stone, Ph.D., of the Faculty of Life Sciences at Tel Aviv University in Tel Aviv, Israel, if more than two-thirds of the population had been infected during the first wave, this would imply that 50% of the 2.2 million people should have experienced reinfection, a figure far higher than the few cases documented by authorities. Furthermore, if there had been so many infections, the specific mortality rate would have been extremely low, 40% lower than that recorded in São Paulo.
Using models that incorporate a flexible basic reproduction number, which changes throughout the epidemic, Stone and his colleagues estimated that during the first wave the attack rate in Manaus was only around 30%, although they were unable to explain why the seroprevalence was so high in the blood donor study. "Our work underscores the difficulties faced by decision-makers when trying to predict herd immunity," they concluded.
Another explanation is that Manaus did indeed have very high infection rates, "but in my opinion, it is not possible to achieve herd immunity through natural infection," Dr. Sabino, who led studies on the outbreaks in the Amazonian city and the emergence of the P.1 variant published in The Lancet and Science , told Medscape in Spanish . "For that, vaccination is required, and after achieving herd immunity, we can return to normal, but with an increased surveillance infrastructure and our response capacity."
5. Herd immunity is essential to end the pandemic.
Some experts believe that herd immunity will never be achieved and that the SARS-CoV-2 virus could become endemic , like influenza or the common cold. "The concept of herd immunity means that once we reach the threshold, the virus will disappear. That's not the case. It's a false notion," said Dr. Jeff Engel, senior consultant for COVID-19 at the Council of State and Territorial Epidemiologists in Atlanta, Georgia, a month and a half ago.
In contrast, the experience of countries like Israel, with at least 60% of its population partially vaccinated, demonstrates that broad immunization coverage leads to a sharp decline in cases, hospitalizations, and deaths. This cannot yet be definitively attributed to herd immunity, although it could be something similar. Some believe that 60% vaccination coverage could be the starting point for an "exponential decline" in cases, even though the virus could still return, at least regionally, according to an article published in Vox.
Expanding coverage to children and adolescents and considering strategies to increase the population's motivation to be vaccinated , especially young people, could be necessary elements to achieve true herd immunity.
But is herd immunity essential to ending the pandemic? "It's actually just another tool," Dr. Daniel Stetcher, head of the Infectious Diseases Division at the Hospital de Clínicas "José de San Martín" of the University of Buenos Aires, in Buenos Aires, Argentina, told Medscape in Spanish .
"The most important thing is to achieve good vaccination coverage and maintain preventative measures. It's important to remember that being vaccinated doesn't make you completely immune; mild cases can still occur and the disease can still be transmitted," he said.
According to Pennington, vaccine-induced herd immunity will be very helpful, but insufficient on its own to rid countries where the virus has become established of SARS-CoV-2, he told Medscape in Spanish.
"Much will depend on the duration of vaccine-induced immunity and the ability of countries to prevent the importation of the virus, as well as to identify and control outbreaks quickly," he added.