Health Tips

Herd immunity, explained

Getting that COVID-19 vaccine jabs offers many benefits: it allows you to see your friends and family members; it protects you from developing serious condition should you be infected by COVID-19; and most important of all, it helps to end the global pandemic by achieving herd immunity in global populations. 

What is herd immunity?

Before we discuss the term ‘herd immunity’, let’s understand two important things:

1. Vaccines, not just COVID-19 vaccines but any vaccines in general, prevent one from catching an infectious disease. Vaccines save millions of lives globally, and they help to prevent unnecessary deaths and sufferings caused by the diseases.

2. However, not everyone is eligible to receive vaccines. For instance, people who are severely allergic to certain ingredients in the vaccines, patients who have immunocompromised conditions and newborns may not be able to receive certain vaccinations. The eligibility to receive a vaccine may vary depending on the type of vaccines.

Okay, now we get these two important premises out of the way, we can finally introduce the concept of herd immunity.

When we talk about herd immunity, we need to think about it in the context of a population or a community. Herd immunity occurs when a high percentage of the community develops immunity to a disease, making the spread of this disease from person to person unlikely. Gradually, the amount of pathogens lingering in the community becomes lesser and can potentially be eliminated, because the human being available for transmitting the pathogens become fewer. As a result, the people who cannot be vaccinated are protected from the disease, even though they did not receive any vaccine. 

For a animated representation of herd immunity, visit https://www.historyofvaccines.org/content/herd-immunity-0

How do we set the target for herd immunity?

Epidemiologists (i.e. scientists who study outbreaks of diseases) can estimate the proportion of a population that needs to be immune before herd immunity kicks in. This threshold depends on the basic reproduction number, R0 (pronounced “R naught”) — the average number of people who will contract a contagious disease from one person with that disease in a population of people who were previously free of infection and haven’t been vaccinated. If you want to get more geeky, the formula to calculate the herd-immunity threshold is 1–1/R0. 

In other words, based on the formula above, we can know one thing: the more people who become infected by each individual who has the virus, the higher the proportion of the population that needs to be immune to reach herd immunity.

How do we achieve herd immunity?

Say we want to achieve herd immunity in a population. Theoretically, there are two ways to do it: 

Plan A: Letting as many people as possible to get infected

Plan B: Vaccination

When epidemiologists talk about herd immunity, they usually talk about it in the context of vaccination. But not everyone shares the same sentiment. During the early phase of COVID-19 pandemic, there were plans in some countries to allow the virus to ‘run its course’ and infect as many people as possible, until a natural herd immunity is achieved. 

Here it’s why we think that it is such a bad idea:

1. There is no evidence throughout human history that this approach would work.

2. Practically speaking, it is impossible to achieve natural herd immunity. In the case of COVID-19, 70% of the population would have to be infected every nine months to achieve natural herd immunity! That is unrealistic. 

3. Natural herd immunity is fragile. Following a study showing that 66% of citizens had been infected with SARS-CoV-2 by the end of the first wave, some concluded that the number of people still vulnerable to SARS-CoV-2 was too small for transmission to survive, meaning herd immunity had been achieved. Those hopes were dampened by a subsequent surge in cases of infection, with exposure rising to 76% in what is now described as “unmitigated transmission.” Also, what if new mutant variant arises, as what is happening right now in the UK and South Africa? Would the natural herd immunity protects us from the new variants? We don't know.

4. There is also a lack of information about how the human immune system behaves with SARS-CoV-2, the virus that causes COVID-19. It is still unclear if antibodies against SARS-CoV-2 will protect people who have been infected fully, or generate wider herd immunity. A study in the UK suggests that the accuracy of the test for SARS-CoV-2 antibodies may be lower than previously suggested, increasing the risk of false positive results. If antibody responses are used as an indicator of immunity, there is a danger that individuals and the government will make decisions based on inaccurate information.

5. Even if natural herd immunity does work, the healthcare system will be crushed to nothing; the natural herd immunity will come at a cost of tremendous sufferings and human lives. This would put a huge dent to not only the economy, but it also means that many people are going to lose their loved ones. Simply put, it is unethical and inhumane to let the disease run its course without actively seeking for solutions.

Plan A won’t work, that is why we need plan B, vaccination. Vaccines introduce a small piece of the pathogen into your body and stimulate your immune system to produce the corresponding antibodies. These antibodies are the soldier on standby mode, and they will protect you should you be exposed to the pathogen in the future. Generally, you can achieve immunity at the expense of some mild side effects such as muscle sore. But hey, you won’t have to go through the sufferings of catching the actual disease. 

Scientifically speaking, vaccination is a smart move to ensure long-lasting immunity; from a moral perspective, it is an ethical thing to do as well. 

Herd immunity can change from time to time

If you have been following the announcements from our dearly Health Director General Tan Sri Noor Hisham Abdullah, you would notice that he mentioned R0 quite often, and those R0 changes from time to time. Since the threshold of herd immunity is determined by R0, it is not surprised that herd immunity is not permanent. 

There are many reasons why herd immunity varies: new babies are born everyday, new individuals join a new population, the emergence of ‘superspreaders’ etc. Even once herd immunity is attained across a population, it’s still possible to have large outbreaks, such as in areas where vaccination rates are low.

Therefore, it is better that we see herd immunity not as a singular point that we achieve at an exact point, but as a gradient. As many variables during a disease outbreak can change, herd immunity does not stay the same. 

Note: Different infectious diseases have different thresholds for achieving herd immunity.

Herd immunity and COVID-19 

The herd-immunity threshold for COVID-19 varies from regions to regions. In Malaysia, the government aims to vaccinate at least 80% of the adult population to achieve a fairly stable herd immunity. Vaccinating a large population may also potentially confer another benefit: it reduces the chance of new variants emergence because the transmission of virus is reduced. Even if the variants did emerge, we can always have vaccine boosters. All in all, to achieve herd immunity for COVID-19 and get out of this pandemic, vaccination is the way to go. 

We’ve also covered the side effects and allergic issues of COVID-19 vaccination, check them out!

Remember to register for COVID-19 vaccination and attend your appointment when the time comes. #LindungDiriLindungSemua

A word from DOC2US

If you have any enquiries related to COVID-19, you can consult our COVID-19 Task Force, which consists of professional doctors and healthcare professionals, for FREE! 

You can access to free COVID-19 virtual health advisory by downloading the DOC2US app on http://onelink.to/doc2us or use our web chat https://web.doc2us.com/

For more information about COVID-19 vaccination programme in Malaysia, visit https://www.vaksincovid.gov.my/

Disclaimer: COVID-19 is a novel disease. The information and scientific evidence of its development and vaccines are changing as we speak. Some content of this article may be outdated in the future. We encourage you to always speak with a healthcare professional you trust for the latest updates on COVID-19 and its vaccines.

References

1. Coronavirus disease (COVID-19): Herd immunity, lockdowns and COVID-19. Accessed March 15, 2021. https://www.who.int/news-room/questions-and-answers/item/herd-immunity-lockdowns-and-covid-19

2. Herd immunity - APIC. Accessed March 15, 2021. https://apic.org/monthly_alerts/herd-immunity/

3. Aschwanden C. The false promise of herd immunity for COVID-19. Nature. 2020;587(7832):26-28. doi:10.1038/d41586-020-02948-4

4. Fine, P., Eames, K. and Heymann, D., 2011. "Herd Immunity": A Rough Guide. Clinical Infectious Diseases, 52(7), pp.911-916.

5. Natural herd immunity should not be used as a means of pandemic control. The BMJ. Published December 17, 2020. Accessed March 15, 2021. https://blogs.bmj.com/bmj/2020/12/17/natural-herd-immunity-should-not-be-used-as-a-means-of-pandemic-control/

6. Alwan, N.et.al., 2020. Scientific consensus on the COVID-19 pandemic: we need to act now. The Lancet, 396(10260), pp.e71-e72.

7. Kadkhoda, K., 2021. Herd Immunity to COVID-19. American Journal of Clinical Pathology.

Cover image credit: Business photo created by master1305 - www.freepik.com

Written by

Ms Joyce Toh