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Do I need to get vaccinated if everyone else is?
Most of us have had many shots in our lives. Vaccines are commonly given against infectious diseases like tetanus, diphtheria and meningitis. More recently, vaccines have been instrumental in reducing the impact of the COVID-19 pandemic, with over 12.7 billion doses being given across 184 countries.
Despite numerous success stories, vaccine hesitancy is still a widespread issue. Scientific misconduct and fraud has unfortunately caused some people to be concerned about the MMR vaccine, which protects children from measles, mumps and rubella. Vaccines only work to prevent disease spread if most people are immunised. How many people need to get the shot before it’s no longer important for more people to be vaccinated? For example, if 92% of children have had the MMR vaccine, do more children need to get the shot? To answer these questions, we asked 23 experts in vaccines, epidemiology and mathematical modelling, “Is it still beneficial to be vaccinated if 92% of the population is already vaccinated?”, here is what they said…
Is it still beneficial to be vaccinated if 92% of the population is already vaccinated?
What is herd immunity?
Professor Martin Maiden, an expert in vaccines and epidemiology from Oxford University in the UK, says “The concept of vaccine generated 'herd immunity', also called 'population immunity' or 'population protection', depends on a vaccine generating immunity that not only (i) prevents an immunised individual developing the symptoms of an infectious disease, but also (ii) prevents the vaccinated individual spreading the disease to others. Having the disease can also provide this immunity against many pathogens, but is much more risky to the individual.”
How many people need to be vaccinated?
Professor Maiden says “The degree of population immunisation required to generate herd immunity is dependent on (i) how effective the vaccine is (none are 100% effective, but most work very well in most cases) and (ii) how easily the pathogen in question spread. For example, measles spreads really well, with around 20 additional cases arising from each infection, therefore even though the vaccine is very effective (i.e. it works in almost everybody) high levels of vaccination are needed to stop the spread of the disease. This is why measles outbreaks occur when vaccination levels fall.”
Epidemiologists can figure out how many people need to be vaccinated against a particular disease using mathematical modelling. Dr Aldo Dekker, an expert in epidemiology and vaccinology from Wageningen University in the Netherlands, explains how this modelling works: “The reproduction ratio (R-naught) is defined as the number of new infections caused by a typically infectious individual in a fully susceptible population. If the R-naught is below 1 an infection will (eventually) die out. Vaccination reduces the number of susceptible individuals. The fraction of individuals that have to be protected against infection should be larger than 1-1/R-naught. In the case of 92% protected individuals infectious agents with a R-naught up to 12.5 would die out.”
Why may vaccination still be important even if many people are already vaccinated?
So far, we have covered two important factors involved in herd immunity: how quickly the disease spreads (the R-naught) and how many people are vaccinated. However, there are many other factors that are important when modelling disease spread, and most of these factors mean vaccination rates may need to be higher than we thought.
One factor is that you see different levels of vaccination rates in different places. Dr Seth Blumberg, an expert in epidemiology and applied mathematics from New York University in the USA, says “Vaccine coverage is not uniform and pockets of susceptibility can put certain people at greater risk than others”. People move between places that have different vaccination rates, and therefore may spread the disease this way. Dr Luis Acedo, an expert in epidemiology from Valencia Technical University in Spain, says “The fact that the world is increasingly connected, and we can not ignore population mobility, means that high vaccination coverages in our country are not a guarantee of herd immunity protection.”
Another factor to consider is that vaccination rates don’t perfectly reflect immunity rates. Dr Anthony DeLegge, an expert in applied mathematics from Benedictine University in the USA, says “Immunization from a vaccine isn't a guarantee. While most people who receive a modern vaccine will develop immunity, this is not always the case, and that immunity could wane over time. This is why we are encouraged, for example, to receive tetanus vaccines every 10 years.”
When discussing vaccines, ‘herd mentality’ could be considered almost as important as ‘herd immunity’. Professor Maiden says “The question of whether one should be immunised or not is a balance of benefit to oneself and one’s community against cost of the vaccine and any risks. In simplistic terms, if everyone else has the vaccine, and therefore and bares the cost and risk, the selfishly rational thing for any individual to do is not to be vaccinated oneself and therefore have the benefit of the vaccine indirectly. This of course cannot really work, because if everyone does this then no one gets the vaccine and no one gets the benefit.”
What about vaccination against measles?
In the UK, MMR vaccine coverage dropped from 90.3% in 2020-21 to 89.2% in 2021-2022. What percentage of the population need to be vaccinated to keep measles at bay?
Dr Michaela Hall, an expert in applied mathematics from UNSW Sydney in Australia, says “The vaccination coverage required to eliminate measles is estimated to be 90-95%, which is consistent with the WHO target of 95%. Even if the UK were able to achieve 95% coverage next year, this is unlikely to provide sufficient protection in the near-term, due to poorer coverage among previous cohorts and the necessity to maintain that rate.”
Dr Michael Mina, an expert in vaccines and infectious diseases from Harvard University in the USA, says “if 92% of the population is vaccinated, those 92% of individuals will be an effective firewall, preventing the infection from spreading and causing disease in the 8% who are not vaccinated. However, if an individual decides that he/she, or their child is protected because of the 'firewall' and therefore has such a low chance of getting the infection that they decide not to bother getting the vaccine, then they do not contribute to the firewall, creating a small crack in it. At a population level, it is likely that their decision was either informed by or will inform other people's decisions to not vaccinate for the same reason. And very quickly, the firewall will contain many cracks because many people think that there is no need.”
Dr Mina goes on to say that “measles is so infectious that it only requires a few cracks to start a new measles outbreak. In more concrete numbers, as soon as the population that is vaccinated falls below about 94%, measles will began to come back. We have seen this throughout Europe in 2017 and 2018, where hundreds of thousands of people have been infected with measles, and in the US as well, where small pockets of unvaccinated people led to numerous small cracks in the firewall, and thus an outbreak of measles occurred. Therefore, unlike other modern medicines, vaccines have to be considered within the context of the larger population.”
Should you be vaccinated?
Professor James McCaw, an expert in epidemiology and public health from Melbourne University in Australia, says “For healthy individuals, vaccination is always beneficial. The only people who should not be vaccinated are those with medically-confirmed issues which make vaccination for them unsafe (for example, immunodeficient people). Given the number of people in the population who fall into categories such as this, the benefits of vaccination almost certainly outweigh the risks for those without these medically-confirmed and vaccine-specific issues.”
Even when vaccination rates are high, it is always important to get vaccinated.
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