Whilst “thinking out loud” in a press conference last week, the UK Government raised the role that herd immunity could play in the fight against the current coronavirus pandemic. It has been estimated that 60% of the population would need to be infected for herd immunity to be achieved, clearly leading to many deaths and so this was understandably met with widespread dismay. Following this, Matt Hancock MP, Secretary of State for Health and Social Care speaking to Andrew Marr, stressed that this is not part of the Government’s plan to tackle coronavirus. But it still throws up questions and one has to ask, if the public actually understand what herd immunity is and if it could realistically be a viable option in tackling the current coronavirus pandemic?
What is herd immunity?
Herd immunity describes a situation where a sufficient proportion of the population has immunity to a given infection such that it slows or prevents disease spread, protecting “at-risk” individuals.
Immunity can be generated through natural infection, allowing the body to mount an immune response to an invading pathogen. However, in this case it means the individual must go through disease to acquire this immunological protection for the future. Alternatively, vaccination can be used which introduces the body to a form of the pathogen that will not cause the disease in the individual but still enables them to generate a protective response in a controlled manor.
Effective herd immunity is typically achieved through vaccination, therefore not only protecting the vaccinated individuals themselves from becoming ill but creating a situation of minimal disease spread whereby anyone who cannot be vaccinated or fails to develop effective immunity after vaccination is protected. At-risk individuals may include those with underling medical conditions or the elderly where immune function may be impaired. For some diseases, children are also classed as at-risk, but this depends on the specific disease being considered.
Prof Rowland Kao, Sir Timothy O’Shea Professor of Veterinary Epidemiology and Data Science, University of Edinburgh, said “Herd immunity is a potentially confusing term because it really has nothing directly to do with the immune system. When everyone in a group (i.e. a “herd”) is susceptible to a disease, and able to transmit it once infected, this means that once anyone in the group becomes infected, then everyone else is at risk. And so the disease has a good chance to propagate. However, if some of the group are protected, for example by vaccination, then this means that at least some of the time, a contact that would have been infectious, isn’t infectious, because the contact was with someone who couldn’t get infected. Because the number of contacts over the lifetime of an infection is limited, this therefore means that the disease’s ability to reproduce is impaired. If there are enough individuals protected so that, on average, the disease when introduced can infect less than one other, this means that the disease will infect maybe a few, but won’t spread broadly through the population. The important point is that not everyone needs to be protected in order for the group as a whole to have little chance of getting infected. The concept is therefore called “herd immunity” because it means that, at the group or herd level, there will be relatively few infections in the group, even if there is at least one infection introduced.”
Herd immunity from vaccination or natural infection
Vaccination is not the only way to generate immunity within a population. People who become infected naturally and recover are likely to develop lasting immunity which could have the same effect as vaccinating the population. There are however a number of points that need to be considered here.
Dose – When administering a vaccine, a controlled amount of the antigen(s) is administered, but in natural infection the dose is likely to be unknown. This may mean someone receives a high level of exposure and may therefore be prone to more severe disease or complications. Alternatively, if the dose is very low, this may impair the effectiveness of a future immune response were they to be exposed to the same pathogen again.
Duration of immunity – Typically, during the development phases of a vaccine, antibody levels will be monitored over time and the duration for which immunity lasts tested. This type of information enables vaccine companies to determine what doses are appropriate and how frequently an individual may require booster vaccinations to maintain protection. With natural infection however, determining the duration of immunity may be difficult and vary depending on other variables such as dose.
How does immunity affect the spread of a virus?
The spread of a virus can be slowed and even halted by the presence of immunity within a population. This immunity can be acquired or artificially induced using vaccines. In this graphic, we look at two populations, one without any immunity and one with immunity, to see how the spread changes.
Population 1: No Immunity
In the image above we see the progress of a virus (in red) though an uninfected population (in black) without any immunity.
The basic reproduction number (R0) is the average number of infections that would arise from a single case being introduced to an entirely susceptible population. In phase 1, the virus, which has an R0 of 3 (meaning every infected person goes on to infect an average of three other people), spreads from an infected single person to three uninfected people with no immunity. In phase 2, those three people now infect three more people each, leading to an increased infected population, seen in phase 3.
Population 2: Immunity Present
In this second image, we see how immunity in the population (immune people are in green) can stop the spread of a virus.
In an immunized population, researchers use a different metric, the effective reproduction rate (R) to measure the spread of the disease. In phase 1 this time, the same virus attempts to pass from a single infected person to three others but can only infect one (phase 2). As only one person is being infected on average, the R is reduced to one.
If R can be pushed below 1, the virus will gradually die out in the population as more people recover than are infected.
This example is for a fictional virus, but measures like R0 and R can be used to assess all infectious diseases that spread through a population. SARS-CoV-2's R0 is estimated at 2.5. There is no known community immunity to COVID-19.
Image credit: Technology Networks. Created with input from Andrew Lee, a medical statistician with an MSc in Epidemiology from the London School of Hygiene and Tropical medicine.
Can herd immunity protect us?
A herd immunity approach can be very effective in keeping disease cases down in a population. Take measles for example. It was shown that 93-95% of the population need to be immune to measles to effectively protect the remaining population who aren’t, be that because they cannot be vaccinated, are immunocompromised or did not mount a protective immune response. In recent years we have seen what that breakdown in herd immunity has done. With falling vaccine uptake, cases soared in the many countries who had not had a measles problem for many years. In some cases, this led to mandatory vaccinations to rectify the situation.
Whilst herd immunity can be effective, it cannot be applied to all infectious diseases. Those in which it can be effective are typically restricted to a single host species within which transmission occurs by relatively direct contact, and infection/immunization induces solid immunity.
Other factors to consider include:
Rate of mutation – With a pathogen that has a very slow rate of mutation, someone with prior immunity through infection or immunization who is then exposed to the pathogen again has a good chance of being protected by their previous exposure or vaccination – the solid immunity mentioned above. For pathogens with higher mutation rates however, such as rhinovirus which causes the common cold, just because you may be protected against one form, may not mean that the new form encountered is sufficiently similar to offer cross protection.
Different pathogen, different dynamics – Factors such as infectivity of a pathogen are important when considering what proportion of the population need to be immune for herd immunity to be effective. For a disease that does not transmit between people easily, fewer people will need to be immune for spread to halt, making heard immunity more achievable. The more contagious the disease however, the higher the percentage of people that will need to be immune for herd immunity to be achieved.
What about coronavirus, is herd immunity an option to protect the vulnerable?
“Protection comes about because in a partially immune population infected individuals are less likely to encounter uninfected ones and so transmit the virus to them. Consequently, infection chains are interrupted, and spread is stopped or slowed. The proportion of the population that needs to be immune for the number of new cases to decline depends on the basic reproductive ratio of the virus, known as R0. This is the average number of secondary cases that arise from each primary case when a virus is spreading in a wholly susceptible population” commented Dr Simon Gubbins from The Pirbright Institute.
He continued “For SARS-CoV-2 estimates for R0 are around 2.5, so the proportion of the population that needs to be immune to achieve herd immunity is around 60%.” However, the coronavirus, SARS-CoV-2, at the center of this current pandemic is novel, which means there is consequently a lack of any pre-existing immunity leaving the whole population susceptible to infection. This is compounded by the absence of an effective vaccine and unknown duration for which immunity may last.
Whilst models for diseases like measles are based on many years of data regarding immunological and disease dynamics and pathogen mutation rate, this is still very much an unknown for SARS-CoV-2, meaning even if you were to achieve what is believed to be the percentage infection required for herd immunity, there is no guarantee this will protect at-risk groups in the future.
“Herd immunity acts as an evolutionary pressure for a virus to adapt so that it can escape immunity and can spread more easily. Influenza viruses are very good at this and frequently mutate to produce new strains to which people are not immune. This is the reason the seasonal flu vaccine needs to be updated annually. There is no information to show whether something similar will happen with SARS-CoV-2” concluded Dr Gubbins.
Considering current strategies being employed to slow the spread of disease, Dr Thomas House, Reader in Mathematical Statistics, University of Manchester commented “Social distancing measures do not lead to herd immunity, so when they are lifted the epidemic may grow again. Whether we aim for it or not, herd immunity will happen at some point in the future since neither a growing epidemic nor social distancing measures can continue forever, and the aim of policy should be for this to happen with the minimum human cost possible.”