Even if we were all vaccinated, Covid-19 would still be a problem

It was many experts’ original expectation that immunity acquired either from natural infection or vaccination, or both,  would result in a state of herd immunity: ie a sufficiently high proportion of the population would be resistant to becoming infected.  Thus, the virus would not have enough unprotected people it could spread infection to.  The pandemic would thus peter out.

For several reasons this is now proving unlikely and even if we were all vaccinated the Covid-19 virus could still be present and being passed from person to person.

In this post I consider what is behind this and what are the implications for individuals and societies.

What is the level of protection from current vaccines? 

  • Well known, of course, but Covid-19 has a spectrum of severity
  • Again, well known, the greatest achievement from vaccination is to reduce the risk of getting severe Covid-19  

Most recent data suggests that, even for the delta variant, vaccines reduce the risk of needing to go to hospital by 19/20 (95%)

Similarly the vaccines probably stop about 2/3 of people getting any detectable infection

How many are getting ill despite being double vaccinated?

  • The above figures show that the vaccines do not prevent everyone who has been vaccinated from having a detectable infection, even if they do not become ill
  • They also show that , albeit rarely, there will be some fully vaccinated people who do become seriously ill
  • The reductions of 2/3 for any detectable infection and of 19/20 for serious infection are important to have in mind as we interpret the news about vaccinated people becoming ill 
  • It is obvious that, everything else being equal, the bigger the number of people in a population who become ill, the bigger the subgroup of those who had been double vaccinated
  • We might then conclude wrongly that the vaccine did not work as well in Green town
  • That would be false because without vaccines the overall number would be much higher in Green town
  • Indeed in the extreme case, if in Green town 100% had been vaccinated, every case of infection would be in those who had been double jabbed  
  • It is not surprising that we will know fully vaccinated friends, relations and colleagues, even TV presenters  who became ill
Andrew Marr is a well known BBC presenter in the UK
  • And that in places with a successful vaccine programme, like Israel, headline data might suggest, as below, that vaccines are not successful 

Why are the vaccines better at stopping serious infection than preventing all infections?

  • The vaccines work by stopping the virus entering cells and this seems to be especially effective for lung cells, which is why vaccinated people are much less likely to get serious breathing difficulties

Why some vaccinated people are protected against being infected at all and others are not?

  • There are a number of possible explanations which include:
    • The amount of virus individuals are exposed to: the more contacts, the closer the contacts and the higher the number of droplets – each would make it that bit harder for the vaccine 
    • Differences in the genes which fight infection: we all have systems to hold off infection and in some people these are more effective than in others
    • A stronger immune response to the vaccine:  we know that people do vary in the level of immunity resulting from the vaccine.  This is not always easy to test (it’s more complicated that can be assessed from a single  antibody level)

Was our original optimism about the vaccines misplaced?

  • If the Covid-19 virus followed the rules, then we had anticipated that there would be enough people who were immune to infection, either because of natural infection, or as a result of immunisation
  • Once that stage was reached the virus would have had no-one else it could infect and would then die out – this is behind the concept of ‘herd immunity’ 
  • We did not know how high ‘enough’ would need to be, it was thought as the pandemic progresses this could be as low as 60%, the pessimists felt it could be closer to 90%
  • What had not been anticipated was the virus’s ability, especially of the delta variant, to be sufficiently capable of evading the immune system in some people so that it can multiply and be passed to others – including  a vaccinated person – though they might be much less likely to become ill

So, does this mean the pandemic will never go away?

  • Vaccines do stop an important proportion of infections and in time this should translate into the pandemic disappearing
  • Remember the all-important R value!
    • It is still correct to say that if each new case infects on average less than one other person, the infection will die out
    • The lower the value of R is below 1, the quicker the pandemic will die out
  • The delta variant is so transmissible that it makes it a real challenge to get R below 1

Trends in R values since the beginning of the pandemic

  • The charts below show the calculated estimates of R over the  duration of the pandemic, in the UK and USA, since the start.
    • The data for August (shown in brown) is the best estimate based on the current data
    • The paler shading around the thicker lines shows that there is still a level of uncertainty as to what the exact values are 
  • As can be seen despite the vaccine programmes it is a real struggle to get the R value below 1

If vaccines are not the complete answer, what can be done?

There are a few possible responses to this:

  • We don’t worry about stopping all infections and just focus on stopping serious infections
    • This can be substantially achieved by current vaccines 
    • There is a continuing debate about a booster jab, but it should be useful in further reducing transmission
    • Ensure the most vulnerable limit their risk of contracting infection, especially in areas/times with high numbers infected 
  • Keep on with some measures of mitigation – mask wearing is the most obvious as it:
    • does little harm to the economy or normal activity 
    • is also the best way to keep transmission low
  • We could reduce transmission by achieving the highest possible rate of vaccination
  • There is a real debate as to the role of vaccinating children to achieve this:
    • Children (especially teenage) are the greatest reservoir of infection that can be transmitted to others
    • It would be challenging to lower R without their vaccination
    • The net benefit to the children themselves from being vaccinated is probably close to zero 
    • It is a novel ethical question as to whether vaccinating children to protect adults is justifiable

My conclusion

  • We can’t get rid of Covid-19 completely any time soon, we will have to live with a high prevalence of detectable infection but, for most vaccinated people, a low risk of being seriously ill
  • Vaccines have made an enormous difference on the numbers getting seriously ill, but cannot stop the virus remaining a threat to some
  • Achieving as high a vaccination rate as possible, including children, could speed up the time to elimination
  • Now is not the time to throw away the masks! 

4 replies on “Even if we were all vaccinated, Covid-19 would still be a problem”

The net benefit of vaccination to children is very high. Children depend on their parents, and sometimes grandparents, for their wellbeing. Reducing the chance of these vital adults getting infected, incapacitated, or killed greatly benefits the child.


Interesting point. The ethical dilemma though relates to the tiny but real risks from vaccination. There is a concern about inflammation of the heart muscle in a very small number of children given mRNA vaccines. So it is not a zero sum game. There is also the global health perspective which even Andrew Pollard (the co-lead researcher on the AZ vaccine) which is: in a world with limited supplies prioritising high risk adults in low and middle income countries over low risk children in high income countries is a moral choice!


Dear Alan, thanks as ever for this thoughtful article – very helpful. The authorities seem always to be worried about further mutations, especially since the structure of the virus is prone to this. Recently I saw comments to the effect that the Delta virus is so transmissible and widespread that it may leave no “room” for further mutations to operate. Please could you comment on this?


Thanks Peter. The interesting thing about this virus was initially how little it mutated, surprisingly. You’re right from an evolutionary perspective there is no need for the virus to mutate further given the relative success of the delta variant to transmit. There are quite a growing number of other ‘variants of concern’ (WHO terminology) but so far none has really spread very far


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