Who should be vaccinated?

A vaccine is now the only likely successful path for the elimination of the threat from Covid-19.  Clinical trials are well underway but results may not emerge until spring 2021.  The UK government* as well as the USA (ie Donald Trump) are considering employing vaccines without waiting for the full trial results.  Vaccine manufacture is proceeding ‘at pace’. What is not clear is whether the vaccine should be offered to those most at risk or the whole population.

*The UK government are considering supporting challenge trials- the issues surrounding these will be discussed in my next blog post

What are the issues?

Who should get the vaccine depends on the following issues:

  • Who is most at risk from the adverse health consequences of infection?
  • What are the risks from the vaccine?
  • How effective is the vaccine?
  • The role of vaccines in achieving herd immunity
  • Population acceptability

These are considered in turn:

Who is most at risk?

  • For annual influenza, most countries only vaccinate the elderly and others with pre-existing health disorders 
  • Covid-19 as is well known poses by far a greater risk to the elderly.  I illustrate this below with mortality data from the UK and USA
  • The chart also shows that the rates of being admitted to ICU and are still tiny for those under age 65
  • Population data on long term ill health, so called ‘long CoVid’, (eg the persistence of symptoms such as being breathless and fatigue) will take some time to accumulate but reports so far suggest that these are not uncommon consequences in the young 

What are the risks from the vaccine?

  • There are short term reactions in the young volunteers in the trials, but these are controlled easily by paracetamol (USA-acetaminophen)
  • At the moment we don’t know the risk of serious side effects such as the recent cases of neurological problems (see my previous blog posts)
  • Based on studies of mass use of flu vaccines, the increased risk of serious neurological problems is very small, say less than 1/million population 
  • As a worst case scenario though, let us suppose that with the Covid-19 vaccines there is an incidence of 1/10,000
  • The red line shows the rate of serious complications of the vaccine at 1/10000.  This could be a wild over-estimate but those who (like me) advocate for high take up of what will be a successful vaccine  need to be able to address the potential consequences of this strategy

How effective is the vaccine?

  • The jury is still out on how successful the vaccines in development will be in preventing infection over a sustained period
  • There is the real possibility, due to waning in the activity of the immune system as we age, that the vaccines will be more effective in those least at risk of a complication or dying from Covid-19
  • The FDA (the USA regulatory body) has a threshold of 50% reduction in infection although those developing vaccines are hoping their trials will yield a 60% reduction: which may or may not be large enough to give herd immunity (see below) 
  • Any reduction in the likelihood of individual protection could influence perceptions of the balance  between the risks and benefits 

Personal protection versus herd immunity 

  • The introduction of a new vaccine gives individuals two paths whereby they can be protected:
    • Individual protection from own vaccination
    • Community protection from others being vaccinated, leading to herd immunity.
  • As shown clearly in the picture below, once the vaccination levels get up to a sufficient size (more blue blobs) then there are fewer routes for an infected person to infect an unvaccinated individual
  • We cannot easily determine what that level of vaccination should be: it depends on the infectivity of the virus (eg R value) and also the current level of natural immunity (from people who had become immune from being infected) 
  • There is a range of suggestions as to what that proportion of the population needs to be, with estimates ranging from 60-80%

“If I’m vaccinated why should I worry about herd immunity?”

Individuals, even those who are willing to be vaccinated, cannot ignore the benefits from herd immunity – why?

  • There will always be a number of people who should not be vaccinated, because of problems with their immune system – either from diseases or from drugs that damp down the immune system.  This group would have to rely on herd immunity for their protection 
  • Some people may have a bad reaction to the first dose of vaccine and in a regime that requires a booster dose, would be left unimmunized 
  • The vaccine may not be sufficiently successful in some people, for example older people
  • Vaccine effectiveness may wane over time, again perhaps more so in older people
  • The damage done to our societies by the virus needs to be controlled and that will require herd immunity to be achieved as rapidly as possible

The anti-vax lobby

Perhaps because of, or in spite of, the issues raised in this post, the size of the lobby against vaccination is substantial and could seriously impact on our ability to achieve herd immunity.

  • Public opinion changes but between 1/3 to 1/2 of the population might refuse vaccination
  • There is a strong view amongst the ‘libertarian right wing’ especially in the USA against vaccination 

Why is there an opposition to vaccination?

  • We need to distinguish between individuals who are opposed to vaccines for themselves and the smaller but more vocal group who are opposed to vaccines for the population. 
  • Concern is that the increased publicity to the latter group will increase the size of the former.
  • There is an established body of research identifying the reasons for opposition to vaccination
  • In a recent large European study, the most prominent single reason for opposition to Covid-19 vaccines is concern about side effects
  • There are other reasons including the perception that:
    • The target disease is not a major risk to public health in general
    • The target disease is not a major risk to them in particular – important in Covid-19 in relation to younger people
    • The vaccine is not effective
    • Governments and/or vaccine companies have dubious motives


  • There is a paradox:
    • Vaccinating the most vulnerable in the population may not be adequate to have the necessary impact on the rate of serious outcomes from this virus 
    • Yet it is going to be a challenge to achieve high levels of vaccine uptake amongst the least vulnerable
  • Any further publicised cases of serious side effects, however rare, could have a disproportionate negative influence on vaccine uptake

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Further reading:

  1. Once we have it, will we use it? A European survey on willingness to be vaccinated against COVID-19. Eur J Health Econ 21, 977–982 (2020).
  2. WHO Influenza Vaccine – Observed Rates of Vaccine Reactions Information Sheet (PDF)
  3. COVID-vaccine results are on the way — and scientists’ concerns are growing (news article):

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