Herd immunity is one of the most controversial topics in the debate around COVID-19. The concept is based on the theory that a community becomes immune to a contagion when most of its members become resistant to the pathogen, either through previous exposure or vaccination. Once a population reaches herd immunity, the rate of person-to-person spread of the virus in the community shows a dramatic decline and outbreaks are managed relatively easily.
In practice, nobody is certain what percentage of a population needs to be immune against the emergent SARS-CoV-2 infection to grant immunity to the community. According to WHO, at least 65-70 percent of the population must have protective immunity for herd immunity to set in. Some epidemiologists, however, peg the figure considerably higher — at 80% or more of the population. On the other hand, there are also those who believe that the threshold for SARS-CoV-2 herd immunity could be as low as 10%, and that many populations are already on the verge of reaching it!
Herd immunity may vary from pathogen to pathogen. Virologists say that the more transmissible the virus, the higher the proportion of the people that need to be immune to it for herd immunity to set in. Measles morbillivirus, for example, can be transmitted to 15-20 people by one infected person. Hence in order to attain herd immunity against this virus, at least 95 percent of the population must be vaccinated. The transmissibility or the contagiousness of a virus is indicated by the basic reproduction number (R0). Compared to the measles virus, SARS-CoV-2 has very low transmissibility — 2 to 3 — with an RO of 2.2. In this case, at least 65-70 percent of the number required for herd immunity can be reached safely and effectively through a vaccine. However, R0 does vary across populations and over time, depending on the nature and number of contacts among individuals in a society and other environmental factors.
On the other hand, reaching natural herd immunity is a long-drawn process compared to the vaccine-based method, in which the most vulnerable groups are targeted first and immunised to curb further spread of the infection. Moreover, natural immunity, once it reaches the herd immunity threshold, need not always be uniformly distributed in a given population. In other words, there is little evidence to suggest that the spread of SARS-CoV-2 might stop naturally after a certain percentage of the population has become immune. Also, no one knows how long the naturally acquired immunity to SARS-CoV-2 lasts. On top of all this, we might end up paying a high price for allowing a free run to an infection with a fatality ratio of 0.3–1.3% to help achieve herd immunity. Models show that achieving an immunity level of even 50% in France and the US through infection would translate into deaths of 100,000–450,000 and 500,000–2,100,000 people, respectively.
Such numbers provide the basis for epidemiologists to recommend nations to resort to all sorts of strategies to stop the spread of the virus, including heavy-handed measures like complete lockdowns.
Sweden provides a rather perfect case study for illustrating the point. This northern European nation is perhaps the only democratic country that stood apart as the rest of the world locked down to contain the virus.
Sweden tried a different approach, focusing largely on places that could turn out to be really dangerous. However, it allowed the opening of primary and secondary schools, restaurants, cafés and shops as usual, with health authorities relying on voluntary social distancing and people opting to work from home. Schools for over-16s and universities are closed and gatherings of more than 50 people are banned, but it was still the most relaxed approach of any EU country.
Today, the number of COVID-19 deaths in Sweden, particularly in care homes, remains among the highest in Europe.
However, as coronavirus cases rise all over Europe in what is feared to be a second wave, the numbers are falling in Sweden. The numbers there are now 90 percent below the June peak and, on a per-capita basis, below those of Norway and Denmark for the first time in five months, show reports.
Swedish authorities believe that natural immunity is at least partially responsible for the sharp drop in cases recently and warn that the situation could be critical in neighbouring countries without such protection.
Unlike other European countries, Sweden expects to be less vulnerable to spikes in coming days as it believes that the population has acquired “a level of immunity that can sort of put the brake” on the pandemic.
Desperate for strategies for moving out of lockdowns, policymakers are leaving no stone unturned. The idea of letting the virus infect people is already starting to look attractive to certain administrations, reports indicate.
A top medical adviser to US president Donald Trump has been urging The White House to embrace a controversial ‘herd immunity strategy’, The Washington Times reported recently.
Even though the report was denied later, critics say that many countries including India are allowing the virus to run its course until much of the population is exposed and the infection rate slowed naturally.
A vaccination-equivalent immunity could be achieved through infection, quite a few seem to think.
But the fact remains that these populations still run risks beyond immediate mortality and immediate morbidity.
That is probably the reason why some experts view the “herd immunity strategy” as a contradiction in terms.
“Natural herd immunity is not a strategy but rather something that builds up gradually as cases occur at reduced rates under containment strategies,” says Gabriela Gomes, Professor of Mathematics and Statistics at University of Strathclyde, Scotland.
Because of heterogeneity in geographical spread, achieving herd immunity is a gradual process, she adds.
Moreover, the term ‘herd immunity’ was not there until a few decades ago, when vaccination programmes were started. It has no place as a strategy to slow a pandemic without a vaccine.
After large waves of infection, communities experience lower levels of the outbreak in most years, and there would be epidemic outbreaks in other years. This cannot be construed as herd immunity; outbreaks will continue to happen all the time, either in the form of a bigger wave or a smaller wave.
It is also better not to infect anyone who doesn’t need to be infected as nobody has any idea what the long-term effects of the COVID-19 would be.
Many viral infections could lead to cancer in later years. The established link between HCV and liver cancer and HPV and cervical cancer are well known.
When juxtaposing the damage caused by further disease spread and the socioeconomic costs of locking down, policymakers must consider how exposing even lower-risk individuals — such as young people with no comorbidities — can still contribute to the spread of the pandemic.
“Trying to reach herd immunity by simply letting the virus spread would come with a catastrophic death toll,” reminds Luis Barreiro, Associate Professor, University of Chicago, Genetics Section.
According to him, prolonged shutdowns are also not an option. But there has to be some middle ground.
The economy can remain open if people respect basic mitigation measures. If everyone wears a mask and respects social distancing, the virus would probably be kept under control. Of course, it is not going to disappear, but infection rates would be manageable until a vaccine becomes available.
Based on what we know about the virus, the key mitigation measures remain wearing a mask and social distancing, avers Prof Barriero.
Increased testing and contact tracing, repurposed and new therapeutics, and vaccination should be the mainstay of any strategy designed to contain the virus, experts insist.
Shadow of reinfection
Recent cases of documented reinfection by SARS-CoV-2 cast a shadow over the prospects of developing a successful vaccine, besides threatening to delay the potential for herd immunity further. A wave of trepidation passed through the COVID-19 research community when they heard reports of reinfection within a relatively short span of time since the virus appeared.
On August 24, researchers from Hong Kong reported the case of a man who had been infected twice, in what is described as the world’s best documented case of a reinfection so far.
When the Hong Kong researchers sequenced the virus from both of the man’s infections, they found significant differences in the viral genome.
Close on the heels of the report came confirmations from two more such cases, from Belgium and the Netherlands, sparking concerns about the emergence of a new variant of the lethal virus.
Epidemiologists, however, say it is too early to sound the alarm as re-infection has been conclusively documented in a very limited number of cases so far.
“Reinfection,” says Prof Barreiro, “so far remains a very rare event.”
Still, if people can become reinfected, complete herd immunity will never be reached and we will always keep seeing sporadic outbreaks of the infection, he adds.
Moreover, we do not know for how long the protection provided by a vaccine will last. In most instances, the protective effect of a vaccine is stronger and longer lasting than that provided by natural infection, further adds Prof Barreiro who is also the lead author of the article titled: Herd Immunity: Understanding COVID-19, appeared in Immunity.
Nevertheless, experts can’t say for sure whether reinfection will remain a rare phenomenon or prove to be a common occurrence. It is also unclear how a history of previous infection would affect the course of the disease in case of a reinfection, and whether some of the pre-existing immunity would affect viral shedding and transmissibility.
In the case of flu pandemics, herd immunity is attained after two to three epidemic waves, each interrupted by the typical seasonality of influenza virus and more rarely by interventions. This is achieved with the help of cross-protection through immunity to previously encountered influenza viruses, and vaccines when available.
Prof Barreiro hopes that we will have a vaccine that will provide long-term protection, and if the protection wanes over time, we would get another boost. “Under such a scenario, and assuming that a large fraction of the population would get vaccinated, the number of new infections should drop dramatically, even if the virus will likely never be completely eradicated.”
Obviously, the development of effective herd immunity may be difficult in the case of SARS-CoV-2. Various factors that makeup herd immunity may be difficult to be achieved simultaneously in all geographies. It may be long before the percentage of immune individuals will reach the threshold required to make the community immune.
An effective vaccine, therefore, can be the safest way to reach herd immunity. Vaccines can specifically target highly exposed segments, such as health-care workers or individuals with frequent contact with customers. Many deaths can be prevented by first targeting highly vulnerable populations, despite the fact that the efficacy of vaccines in older people is comparatively low. In addition, vaccines may be substantially better at reducing viral circulation compared to naturally acquired immunity.