Dear Editor,
Most people I’ve spoken to think about herd immunity all wrong. Thinking of it as a single proportion of people to be vaccinated is not best. To get through this pandemic and deal with a virus here to stay, it needs to be understood what it truly means to achieve herd immunity and, keep it.
The concept of herd immunity began with cows. It first appeared in print (1916) in works by American veterinarian Adolph Eichhorn and other researchers where they noticed that herds of cattle who became infected and survived an infectious disease became collectively immune. Through the years this term made its way into epidemiology where, based on parameters that can change, a threshold number (to be vaccinated) can be calculated to achieve this collective immunity in a given population (at a given time). It’s when immunity levels are high enough that person to person transfer is unlikely and, not only are individuals with immunity protected from the disease but so are people without any immunity because virus suppression is maintained. The threshold number required to achieve this varies for different diseases and its calculation is complicated with a changing situation such as the emergence of new variants/strains, enhancement in transmissibility and of course human behaviour (e.g., people refusing vaccines).
Early on in the pandemic there was a lot of talk about achieving herd immunity as an end. Threshold values quoted ranged from 20-90% and these values kept changing as alpha, beta, gamma, delta and omicron variants of concern emerged (and whatever else will come if enough of the world isn’t vaccinated and immunized). Early on in the pandemic, given that Severe Acute Respiratory Syndrome Coronavirus 1 or SARS-CoV-1 (the nearest human coronavirus relative of SARS-CoV-2, with 86.9% genomic similarity) had spread to 29 countries/regions (during the 2002-2004 SARS outbreak) but was contained as result of public health measures, in my opinion, it wasn’t unreasonable to think that SARS-CoV-2 could follow the same story with public health intervention. Two years into the pandemic we know it’s not the case with this new SARS associated Coronavirus. We are in a dynamic situation as the virus evolves and worse forms emerged because we were not able to achieve high enough immunity globally at the right time.
Of course, aspiring to get most vaccinated in order to achieve a basic level of collective immunity in our communities is a must; mass vaccination is the safest way do achieve this. Vaccines remain at the forefront of virus countermeasures however to continue to focus on a single number of people that needs to be vaccinated as an end to the pandemic significantly downplays the dynamic nature of our situation as well as the non-simplicity of achieving collective (herd) immunity; right now, with global infection surges driven by the Omicron variant every countermeasure adds a layer of protection very much needed. Even if the elusive herd immunity is possible, sustaining it is another story. Take the highly infectious measles for example, the US CDC declared its elimination (no continuous transmission for more than 12 months) in the US in 2000 with herd immunity achieved. However, since then, there have been several outbreaks and the resurgence of measles with a decline in vaccination as parents succumbed to misinformation and stopped vaccinating babies and young children (with no immunity). This exemplifies the difficulty in maintaining herd immunity in order to keep the disease ‘eliminated’. Smallpox is the only disease ‘eradicated’ but this required a global vaccination campaign which lasted decades; it took 184 years from the development of the first vaccine (1796) to it being declared eradicated in 1980.
The point is herd immunity isn’t a state where we have a fixed number of people vaccinated and that’s the end of a disease. It’s when we have developed and maintained a certain level of immunity in the population where person to person transmission is unlikely; this may require multiple doses of vaccine, boosters and years of continuous vaccination depending on how the virus evolves, how long immunity lasts and how people respond to public health measures. With the very contagious Omicron, transmission is high and herd immunity is elusive since Omicron can evade basic two-dose vaccine protection and as such boosters are needed. The conversation (and policy) needs to move towards dealing with a virus here to stay, likely on its way to becoming endemic. To be clear, right now, SARS-CoV-2 is not endemic (it’s not the common cold or the flu which are caused by other endemic viruses). Endemic means, on average, there is a steady level/amount of the virus circulating with risks of outbreaks and not necessarily causing mild disease; outbreaks may be seasonal like Influenza. It does not mean exponential growth as seen with Omicron. So even if overall (on average) disease outcomes from Omicron are less severe than the Delta variant we are not at an endemic stage where we can just live with it. Not yet. We still need to:
1. Continue to maintain focus on vaccinations in order to increase population level immunity.
2. Continue to assess vaccination and infection rates at the community level to determine and communicate disease risks (as it changes) since disease spread starts locally.
3. Continue to maintain and adjust basic public health measures according to disease risk. This is particularly important in places where people are concentrated and transmission is likely to be high.
4. With multiple vaccines and vaccine combinations in use, generate effectiveness data as this would help to track performance and better able to inform policy (locally).
5. Improve access to sustainable supplies of proven and approved COVID therapeutics/drugs for use in worse cases.
6. Genomic surveillance to track variants and better inform policy (see letter, SN April, 26th, 2021). It has been an invaluable tool to help make decisions as worse forms of the virus emerge.
7. Empower and educate people to inform their local communities on the power of evidence-based practices in dealing with infectious diseases (like vaccination). Science and health communication is a continuous process, in a dynamic situation like this pandemic and, as more scientific data becomes available and we understand the virus and disease better.
Sincerely,
Jacquelyn Jhingree, PhD
