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Why are fully vaccinated people still catching COVID-19?

This article was published on
July 27, 2021

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As the Delta variant becomes dominant around the world there are both international and Australian reports of fully vaccinated people still becoming infected with COVID-19. These so-called 'breakthrough infections" have raised questions about the effectiveness of the vaccines. Data from some countries with high vaccination rates are also now showing more weekly cases of COVID-19 in vaccinated people than in the unvaccinated - does this mean the vaccines aren't working? Below Australian experts respond.

As the Delta variant becomes dominant around the world there are both international and Australian reports of fully vaccinated people still becoming infected with COVID-19. These so-called 'breakthrough infections" have raised questions about the effectiveness of the vaccines. Data from some countries with high vaccination rates are also now showing more weekly cases of COVID-19 in vaccinated people than in the unvaccinated - does this mean the vaccines aren't working? Below Australian experts respond.

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Professor Ian Marschner

Vaccination does not eliminate infection but does dramatically decrease severe infection and death. Countries with high vaccination rates, such as Israel, will logically have more infection among vaccinated people because most people are vaccinated. Indeed, if a country has a 100 per cent vaccination rate then 100 per cent of infections will come from vaccinated people. This does not mean that the vaccine is not working. On the contrary, it means that many hospitalisations and deaths are being prevented.

Associate Professor Paul Griffin

Breakthrough infections should not be seen as taking away from the benefits of these highly effective vaccines. While we know as a generalisation protection from death is near 100 per cent, and symptomatic infection in the order of 70 to 90 per cent, they do also reduce the chance of getting infected.

Given it's not 100 per cent however, by definition this means people who are vaccinated can still be infected. These breakthrough infections are likely to be much less severe than they would have been had the individual not been vaccinated and they are also less likely to pass it on to others.

It can be confusing when the number of people vaccinated goes up, the number of cases in people fully vaccinated will also go up even though it's much less than it would have been had the vaccination rate not been high and as a proportion it's much lower.

People who are fully vaccinated in some countries are also a little more likely to get infected as they are very rightly allowed more freedoms than their unvaccinated counterparts. They also may have a reduction in their perceived risk and may then be less likely therefore to use other strategies to protect themselves such as social distancing, hand hygiene and wear masks for example.

While it would be ideal of course to have a vaccine that prevented 100 per cent of infections, this is unlikely to ever be possible (although there is work ongoing to develop vaccines that may do this at least a little better), so it’s likely that in addition to vaccines such as the highly effective vaccines already in use, we will require some additional basic mitigation strategies such as mask wearing, hand hygiene, social distancing and high rates of testing to ensure we can live with the virus and minimise its impact on our day to day lives and livelihoods.

·         What are breakthrough infections?

Unless a vaccine prevents transmission 100 per cent of the time (which is essentially not possible, the polio vaccine perhaps comes the closest), then it remains possible for vaccinated individuals to still get infected. These are known as breakthrough infections. It is important to understand however that this does not represent a failure of the vaccine for many reasons and that even if breakthrough infections do occur, the probability of catching the infection is still less in people who are vaccinated as compared to those that are not.

While breakthrough infections occur, fully vaccinated individuals;

Are less likely to get symptomatic disease

Much less likely to get severe disease

Almost completely protected from death

Are also less likely to pass it on to others

An important paradox to understand is that while it remains clear that fully vaccinated individuals are less likely to catch the infection, once the proportion of vaccinated people in a population goes up then it is likely that more cases will occur in vaccinated individuals than unvaccinated. The rate of infection in those that are vaccinated will remain lower than in those that aren’t but because there are so many more people who are vaccinated, that even a lower rate will end up with higher numbers in this group than those that are not vaccinated. This is not evidence the vaccine doesn’t work, its just that the denominator has been changed.

When comparing infection rates between vaccinated and unvaccinated individuals we also need to take into account changes in behaviour between the two groups. Once people are vaccinated, very rightly their perception of risk from the disease will be reduced hence they are less likely to keep up all of the other strategies to protect themselves. We are also seeing many countries correctly permit additional freedoms in vaccinated people so they are more likely to be able to move around and interact with more people. This means they actually increase their risk of acquiring the infection relative to their unvaccinated counterparts meaning that if they were not protected by the vaccine, they would actually have a much higher rate of infection and the complications that follow from this.

·         What is the real world data from countries with high vaccine rates showing us about the vaccines abilities to prevent all infection compared with severe infection/death?

The efficacy and effectiveness mostly discussed relate to symptomatic proven disease. Not because this is the only effect of the vaccine but mostly because these were the primary outcomes measured in the clinical trials. Reductions in infections are harder to measure so for some time after the promising results from the vaccine clinical trials started to come through, we still had to say we didn’t yet know how much the vaccines would reduce infection. Unfortunately, some took this as saying the vaccines did not prevent infection, however this is not the case. The way we now know these vaccines reduce infection includes;

Animal studies that showed reduced infection rates and reductions in the amount of virus produced in animals that were infected, some of these were close to 100%.

Household transmission studies where rates of transmission are compared between vaccinated and unvaccinated individuals, these are typically showing reductions in the order of 40 to 60%

Measuring asymptomatic infections, these are reduced in the order of 60 to 90%

Measuring the amount of virus able to be detected in infected individuals, reductions in viral shedding are a useful surrogate (not definitive proof of) reduced transmission, various studies have indicated the vaccines do this in the order of perhaps 50 to 67%.

It also makes sense that if we see fewer or shorter durations of symptoms in vaccinated people, and some of those contribute to the ability to transmit, such as coughing, then vaccinated individuals are less likely to transmit

·         Do breakthrough infections mean Australia will need to keep restrictions in place?

Given the vaccines do not prevent infection 100% of the time, it is very likely we will need to continue some mitigation strategies for the foreseeable future. What this may mean is that we can increase the impact of vaccination in protecting susceptible people in the community from becoming infected, by also continuing some of the basic fundamental mitigation strategies we have seen already used with great success before we had access to vaccines. This includes social distancing, hand hygiene, getting tested and isolating until you are well if you have any respiratory symptoms as well as using masks particularly in high risk settings such as plane travel or for people that can’t be vaccinated for whatever reason. So while we are not aiming for eradication currently, with simple mitigation strategies that should have minimal impact on peoples day to day lives the expectation should be a high level of control. If enough people are vaccinated, harsher measures such as domestic border restrictions and lock downs should no longer be required.

·         Will breakthrough infections lead to the creation of new variants of concern?

For the mutations that result in new variants to occur, the virus has to be reproducing in a host, ie infected people. The fewer susceptible hosts in the population, the less likely these mutations are to occur. Certainly, breakthrough infections means that the risk of new variants arising is likely to never be reduced to zero, but the more people vaccinated, the less opportunities for new variants to arise there will be.

·         What do breakthrough infections mean for how Australia will live with COVID in the future?

In my opinion this means that we do need to learn to live with this virus. We should aim to control it as best as possible so the impact of the virus itself, and the mitigation strategies required to control it, have the least impact on our day to day lives. I think the best way to do this is to have the vaccination rate as high as possible and permit people who are fully vaccinated to enjoy additional freedoms they deserve given they are protected. We will then also have to use some basic strategies to further reduce transmission including hand hygiene, social distancing, masks and high rates of testing so we can track where cases are and reduce their opportunity to spread the virus further. This, with better treatments that are hopefully also not too far away, should mean while we will always have to be on guard, we can hopefully gain the upper hand and minimise the impact of this virus moving forward.

Dr Roger Lord

Infections with COVID-19 are on the rise again in many parts of the world. These breakthrough infections are not a failure but rather a limitation of the available COVID-19 vaccines, none have 100 per cent efficacy.

Additionally, no correlates of protection (level of antibody concentration and/or T cell activation required for protection) currently exist to indicate what concentration is required to indicate long term protection, whether this was achieved in a given individual or how long this will last. Vaccination itself does not equal immunisation.

It is not surprising that in countries where higher vaccination rates exist that more vaccinated than unvaccinated individuals have contracted COVID-19. If for example, 80 per cent of a population is completely vaccinated and 20 per cent unvaccinated then statistically more vaccinated people may contract COVID-19.

The important message is that while an individual may still contract COVID-19 following a vaccination regime, the symptoms experienced will not be as severe and less likely to cause hospitalisation.

This is certainly the case in the United Kingdom (UK) where vaccination rates have been high.

The number of COVID-19 cases in the UK is rising due to the emergence of more transmissible delta variant however the number of individuals requiring hospitalisation remains low among vaccinated individuals.

In Australia where vaccination rates are still relatively low, transmission of COVID-19 is still high and will not drop until a greater percentage of the population is vaccinated.

Once a significant percentage of the population is vaccinated frequent lockdowns will be a thing of the past. Until that happens individuals will need to continue to wear masks to help stem transmission of the virus widely in the community.

Professor Nikolai Petrovsky

'Just how good are the current Covid-19 vaccines?' everyone asks. This is actually a next to impossible question to answer, and would require a large thesis to even try to start. The problem is there are just too many variables, from the way effectiveness is being measured, to the population being vaccinated, to the actual vaccine technology being used, to how much virus and which variant is circulating in relevant parts of the world, to what other measures of virus control such as masks and social isolation are being employed, to just what season each hemisphere is in right now.

What we do know without question right now are two very important things - existing vaccines are less effective against the virus variants than they were against the original Wuhan strain and existing vaccines are a long way from being able to completely block virus transmission. This is not to say that existing vaccines do not have a benefit – they clearly do play a major role in reducing hospitalisations and death, just as seasonal influenza vaccines do.  But current Covid-19 vaccines are not perfect, just like current seasonal influenza vaccines are not. Just as we continue to look for better influenza vaccines, so we need to keep looking for better Covid-19 vaccines.

The issue is not the vaccines, it is the public's and politicians' misperceptions of what vaccines can and can’t do that is the problem. A lead example would be the logic (or lack thereof) of vaccine passports. The concept of a vaccine passport is that people who have been vaccinated are safe to others and hence should be able to move around more freely, travel between countries and not have to quarantine or wear masks etc. There is no scientific basis for this as this presumes that the current vaccines completely block infection and transmission which we know they do not.

The same goes for mandatory vaccination of nursing home staff and quarantine workers – while this might reduce their own risk of serious illness if they get infected it provides no assurance that they won’t still transmit infection to those they are caring for, which is the implicit reason for mandating their vaccination. Indeed by increasing the rate of asymptomatic infection, this may paradoxically increase rather than decrease the risk of the virus being transmitted into aged care facilities by asymptomatic carriers.  It should not be surprising that the public gets confused when such scientifically ungrounded policies are introduced either purely for political purposes or because members of our government are themselves totally confused as to the science underlying such vaccines. This is why we need to emphasise the importance of STEM subjects in all curricula.

Professor Jaya Dantas

What are breakthrough infections?

Despite the high level of vaccine efficacy, a small percentage of fully vaccinated persons (i.e. received all recommended doses of a COVID-19 vaccine) will develop symptomatic or asymptomatic infections with SARS-CoV-2, the virus that causes COVID-19. These are called breakthrough infections.

What is the real world data from countries with high vaccine rates showing us about the vaccines abilities to prevent all infection compared with severe infection/death?

Real-world community data from Israel, UK, Scotland, USA, and Chile, all countries with high vaccination rates, indicates that the severity of breakthrough infections is much lower, with low hospitalisations and deaths.

Do breakthrough infections mean we will need to keep restrictions in place?

We will need a strategy with planning, cautious and measured approach to opening up as there are serious implications to continuing with restrictions first and foremost – psychological implications of extended lockdowns, harsh border restrictions, economic implications and we need a phased approach with increase in vaccination rates for all adults and possibly young adolescents.

Will breakthrough infections lead to the creation of new variants of concern?

We know that viruses mutate, and evolve into new variants whether the variants that emerge become variants of interest or concern will need to be monitored and countries need the capability to do genomic testing. Virology institutes around the world are testing this.

What do breakthrough infections mean for how Australia will live with COVID in the future?

Breakthrough infections mean for Australia, all of us have to do our part and get vaccinated. Vaccination protects the individual, and it protects others around us, too.

The most important thing the Covid-19 vaccines do is to protect against hospitalization and death. They have proven to be remarkably effective at this, even with the Delta variant. Breakthrough infections are to be expected, and the more infections occur among the unvaccinated, the more infections there will be among the vaccinated.

The key to stopping the pandemic is for us to reach a high enough level of immunity that the virus has nowhere else to go. We can get there — but all of us have to do our part and get vaccinated. Our rates of vaccination with both doses is still low, especially among young adults.

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