These two words are allegedly synonymous with genocide in 2020. Surreal to say the least.
The ability of the human immune system has ensured survival of the human race. Today however, the Director-General of the World Health Organisation stated that “Never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic“. It’s hard to imagine that he doesn’t understand the dishonesty in his statement. As Martin Kulldorff has stated, herd immunity is no more a “strategy” in public health, than gravity is a strategy in aviation. Just as pilots use gravity to do their job, so do public health programs use herd immunity.
Every disease is unique, so that the concept of herd immunity differs with different pathogens. For example, there is no herd immunity for HIV, a virus which attacks the immune system and makes us vulnerable to multitudes of viruses, bacteria, fungi and parasites. Immunity against HIV is very rare and in 40 years HIV vaccine development has not been successful. There is no herd immunity for TB even though there’s a vaccine. The TB vaccine reduces severity of disease in very young children but it does not prevent transmission of disease. This vaccine took more than 15 years to develop (between 1905 and 1921) and has not been improved upon much in the century since. TB remains one of the main infectious threats to human life on the globe.
The fastest development of any vaccine to date, was Mumps vaccine which apparently took five years. This is an important consideration in claims that we must continue to lockdown our world “until a vaccine is developed” for Covid. This may quite clearly never happen, especially not a safe and effective vaccine.
Herd immunity for Measles requires that around 95% of people in a community are immune (by infection or vaccination), to protect 100% of that community. By comparison, Pertussis requires about 80% of people to be immune. Whilst Measles infection or vaccination confers lifetime immunity, Pertussis infection or vaccination confers temporary immunity. Evidence is now showing that herd immunity against the SARS-CoV-2 virus can occur with much lower rates than required for Measles or Pertussis.
Decisions about who should be vaccinated and who is safe to risk infection with vaccine peventable disease are made in public health all the time. In Australia we use the “HALO” acronym to help with this decision making process, being Health, Age, Lifestyle, Occupation. For each vaccine preventable disease the significance of these factors is quite different. Vaccination against Pertussis is recommended repeatedly throughout our lifetime, but changes according to risk. Newborns and very young children are at greatest risk of severe disease so the vaccine schedule is age targeted towards them. Parents and other adults, including the elderly, with newborn children at home are recommended repeat Pertussis vaccination as a way of “coccooning” young children with household immunity. However the elderly who have no close contact with newborns are not recommended Pertussis vaccination because their risk of disease, and of spreading disease to the vulnerable, is not considered significant. This is a public health intervention which focuses on targeted herd immunity.
The idea that herd immunity is not a consideration in outbreak responses, as announced by the Director General of WHO, is completely incorrect. Another example is Hepatitis A which often causes outbreaks. We don’t vaccinate people who are considered immune to Hepatitis A from natural infection. Hepatitis A does not feature in most vaccination schedules throughout the developing world because it is an infection which spreads quickly where living conditions are crowded and unhygienic. The young in these communities, when infected with the Hepatitis A virus, tend to be asymptomatic or only mildly symptomatic. Their robust response to infection leads to lifelong immunity. This is a clear example of natural infection conferring herd immunity to control outbreaks.
There are multiple other examples and many infectious diseases are not vaccine preventable, including around 200 respiratory viruses which circulate amongst us. The advice to stay at home when you are sick is common sense for multiple reasons. Influenza is one of the few respiratory viruses for which a (very imperfect) vaccine exists. Vaccination is recommended for high risk groups only, and is known to only confer around 15% to 40% immunity amongst those vaccinated in any given year. Yet targeted vaccination, combined with natural infection among those not vulnerable to severe consequences, confers some level of protective herd immunity. Young and healthy adults exposed to Influenza often have very mild symptoms despite the fact that it holds a risk of serious and potentially fatal infection, particularly for children under 5yo as well as the elderly. The 1918 Spanish Influenza pandemic occurred decades before the first Influenza vaccine was developed. This pandemic, which killed an estimated 50 million people globally, ended within 2 years due to herd immunity conferred by natural infection.
Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine, wrote about herd immunity as follows:
‘Herd immunity’ has been reached during previous epidemics of influenza, measles and seasonal coronaviruses. But it’s subsequently been lost (and then regained). What are some of the reasons for this?
Here we’re using technical definition of ‘herd immunity’, i.e. sufficient immunity within a population to push R below 1 in absence of other control measures. But reaching this point doesn’t mean R will stay below 1 forever. Here four things to be aware of…
A: Population turnover. Over time, new births mean an increase in % of population susceptible. This will eventually lead to R>1 and new (but smaller) outbreaks – the more transmissible the infection, the sooner this recurrence will happen.
B: Waning. For seasonal coronaviruses, immunity can also wane (e.g. https://nature.com/articles/s41591-020-1083-1 & https://nature.com/articles/s41467-020-18450-4). Again, this will result in increase in susceptibility over time, and hence potential for R>1 in future.
However, it’s worth noting that subsequent infections for seasonal coronaviruses can come with reduced severity, so although transmission still occurs, burden can be lower during subsequent epidemics.
C: Evolution. In the case of flu, antigenic evolution can mean immunity built against recent strains doesn’t fully protect against new ones (https://nature.com/articles/nrmicro.2017.118). Note: there’s no evidence this is influencing SARS-CoV-2 dynamics so far (https://theconversation.com/coronavirus-mutations-what-weve-learned-so-far-145864).
D: Population migration. The immunity profile of a population may change over time as susceptible individuals arrive/leave. E.g. in 2014/15 there was mass cholera vaccination in IDP camp in South Sudan; by 2016, over 80% were susceptible (https://journals.plos.org/plosntds/article?rev=1&id=10.1371/journal.pntd.0006257).
The COVID-19 pandemic will eventually end because of accumulated immunity, either following infection or – ideally – from future vaccine. But even if this means R<1 initially, the above shows there are number of long-term possibilities to consider.
Herd immunity is a complex concept that can’t be covered in a simple brief blog and I am sure important points are missing here. Nevertheless, anything has to be better than the *odd* (at best) public health messaging from the Director General of the World Health Organisation who, with advisors at hand, surely knows better?
Meanwhile, the STOP TB Partnership continue to sound the alarm on rising rates of Tuberculosis, as one example of many public health problems that are being overridden by the global pandemic response. Progress against the global TB targets, which was already too slow, has been further stalled by COVID-19. Governments must step up now if they are to fulfill the commitments made at the United Nations High-Level Meeting (HLM) on TB in 2018. Two years on, the global TB response is less then 50% funded, with under US$ 6.5 billion available in funding for TB prevention, diagnosis, treatment, and care, despite world leaders having agreed to provide US$ 13 billion per year.