As we approach the end of the whirlwind year of 2020, the Covid-19 pandemic grinds on. Despite increasing pandemic fatigue, record highs in infection rates make it clear: we may be getting tired of Covid-19 but it’s not getting tired of us. As governments around the world grapple with difficult decisions balancing public health and economic needs, some have suggested a different approach: isolate the vulnerable, eliminate pandemic restrictions for healthy people and simply let the virus take its course. Countries like the UK, Sweden and the US, have pointed to the epidemiological principle of Herd Immunity to as a possible path to becoming Covid free. But what exactly is Herd Immunity, how does it work and is there any scientific basis to this claim?
A crash course in epidemiology: R0 and Herd Immunity
Understanding herd immunity starts with understanding the concept of R0 (pronounced R nought), the single most important feature in predicting the spread of any infectious disease. For a given disease, R0 is the average number of individuals that a single infected individual will go on to infect. For Covid-19, estimates at the moment put R0 roughly between 1.5 and 4. Taking an estimate somewhere in the middle of this range, say, R0=2, this means that each infected individual infects on average two others, causing the number of infections to double, and then double again when those newly infected individuals infect others. A disease with an R0 of 1 will not increase in the population, as each infected person only transmits it to one other before they recover or succumb to the disease, so numbers of infected remain stable over time. If R0 is below 1, numbers will actually go down in the population over time:
R0 therefore, has a massive impact on how fast an outbreak will spread. Luckily, the value of R0 isn’t fixed, and public health interventions can work to bring it below 1, driving out the disease. This is done by reducing the number of vulnerable individuals that each infected individual comes in contact with. This is why social distancing, masks and good hand hygiene are so important: they not only protect you and others around you, but this effect is multiplied over time. By breaking chains of transmission, we actually bring down R0 and help steer the disease towards extinction, saving many more people from becoming infected:
The second way diseases become eradicated is through immunity - either natural immunity from contracting and recovering from the disease, or through vaccinations. As more members of the general population become immune, the number of vulnerable people each infected person comes in contact with goes down, reducing the rate of transmission.
After a certain percentage of the population is immune, R0 dips below 1, and the disease gradually dies out, even though a fraction of the population remains vulnerable. This is what we call herd immunity: a state where enough of the population is immune that it protects everyone from the disease.
The concept of herd immunity is used for determining things like the proportion of the population that needs to be immunised for a given disease to be kept under control. For a highly infectious disease like measles, about 95% of the population have to be immunised to achieve herd immunity. For Covid-19 our best estimates at the moment suggest we would need immunity in roughly 60-80% of people- though estimates vary. Proponents of herd immunity as a solution to Covid-19 suggest that if we allow Covid-19 to spread freely among low risk individuals, we would eventually achieve herd immunity and then the vulnerable will be able to come out of isolation. However, this notion has been strongly rebuked by most infectious disease experts, on both ethical and scientific bases. Here are the main reasons.
Why natural Herd Immunity isn’t an evidence-based, practical or acceptable solution to Covid-19
1. It will cause “Untold death and suffering”
The WHO estimates that as much as 10% of the world’s population has already been infected, leading to more than 1 000,000 deaths and many millions of people who now live with long term health impacts. While 10% infection is much more than official numbers suggest, this is a long way away from the estimated 60-80% needed for herd immunity. This means many, many more cases and deaths before herd immunity is achieved. Further, while most have been working to flatten the curve, removal of restrictions will do the opposite, causing rapid spread and sudden infection spikes, causing hospitals to become overwhelmed as the severe cases inevitably rise. This means more avoidable deaths and illness as less care will be available to patients of Covid-19, and many other sick and healthy people who require health care for things like pregnancies, and routine check-ups.
2. The level of infection matters
Some evidence suggests that the level of initial exposure to Covid-19 may affect the severity of symptoms. The idea is that low levels of exposure, the body has more time to mount an immune response before the viral load becomes very high. Therefore, if someone takes measures to protect themselves, and still get infected, those precautions could still aid their recovery. Eliminating restrictions and allowing “healthy” people to mix freely could lead to individuals being inoculated with higher initial doses of the virus, with the potential for more severe infection.
3. We can’t always tell who will be vulnerable
The strategy relies on identifying vulnerable individuals based on age and underlying health conditions. Yet we know that healthy individuals sometimes do die of Covid-19 - this would be an inevitable byproduct of this strategy. Second, many individuals have undiagnosed underlying conditions that would not be easily identified.
4. The most marginalised in our society will be left behind economically
We know that some of the most vulnerable communities to severe Covid-19 are the elderly, the disabled, those with underlying health issues and in the case of the US and parts of Europe, ethnic minorities. This strategy puts the burden on these communities to protect themselves from Covid-19 by isolating themselves from society and economic activities, while the young and healthy forge ahead. It means that instead of making workplaces safe for all through protective measures like physical barriers, social distancing and remote working, an individual with underlying conditions has to choose between their life and earning an income, and by choosing their health, are simply excluded from job opportunities. A strategy that disproportionately victimizes the vulnerable should never be the solution to a shared problem - we are facing a collective threat, and the burden should be shared collectively.
5. It just won’t work
Proposals of herd immunity for Covid-19 have been frustratingly vague. Key questions on how we identify and isolate the vulnerable and how do we mandate compliance have not been addressed. Further, a fundamental assumption of the strategy is that people who get Covid-19 have long term immunity. This has not yet been confirmed. Just this week a study found that Covid-19 antibodies drop just a few months after infection dealing a blow to this narrative. Even if there is immunity, scientists say that we likely won’t get to the level of infection needed for herd immunity, or at least not for a long time. Even in places which have seen massive outbreaks like Spain and Italy, a recent study has shown that the percentage of people with Covid-19 antibodies are in the single digits, raising to 10-15% percent in heavily infected urban areas.
Perhaps the fact that drives this home, is that we have never achieved natural herd immunity for a novel virus. Consider smallpox, a disease which has been with us since at least the 3rd century BCE and was a major cause of death for centuries globally. We didn’t achieve herd immunity to smallpox until 1980 after an extensive vaccination drive by the WHO. The reasons it’s so hard to have extremely high levels of immunity naturally is that for some diseases immunity wanes, and where it doesn't, not all individuals become immune at the same time. There are always new vulnerable individuals being born into the population and isolated groups that are naïve to the disease- for example the native populations of the Americas which were decimated by smallpox after colonisation by Europe.
What should we make of this?
It is clear that compromises must be made to safeguard public health and economic needs. However, these compromises should be based on understanding the scientific evidence, not picking and choosing concepts based on predetermined political agendas. Early in the pandemic ideas of herd immunity were suggested, but the increasing consensus among scientists and experts is that this is not a viable solution. The danger of these ideas is they shape the public perception during an uncertain time when we rely on leaders for guidance and expertise. Please take the advice of evidence-based public health experts in protecting yourself, keep yourself informed and take the steps you need to take to keep you and your loved ones safe.
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