Why are fully vaccinated people getting infected with COVID-19?

Last Tuesday, a fully vaccinated nurse at a COVID ward at Westmead Hospital in Western Sydney tested positive for COVID-19. As well as receiving both doses of a COVID vaccine – it is not clear which one – she had been wearing full PPE.
 
Later in the week, a similar story emerged in Queensland where a woman employed at Brisbane Airport returned a positive result for the virus – again after receiving two doses of a COVID vaccine.
 
Such reports are only likely to become more frequent as the vaccination rate accelerates across the country.
 
The picture abroad, where more people have had their full vaccine dosage, is a clear bellwether. The London-based Financial Times reported that around 17% of recent infections caused by the Delta strain in the United Kingdom are among those who have been fully vaccinated (defined as more than 14 days after the final vaccine dose).
 
The statistics have, almost inevitably, prompted conspiracy theories suggesting that vaccination is not effective – including one recently sparked by an unfortunate slip of the tongue from a health official in New South Wales and now thoroughly debunked by AAP FactCheck.
 
For Professor Robert Booy, a vaccine expert at the University of Sydney, the key figure is not the number of breakthrough infections affecting vaccinated people, but the level of protection the vaccines offer against chronic illness and death.
 
He describes the vaccines’ impact on symptomatic infection as ‘substantial but not high level’.
 
‘The immune response to vaccines is strong, but the vaccines are not sterilising, they don’t completely prevent infection,’ he told newsGP.
 
‘[Being fully vaccinated] gives you moderate protection against infections that are milder like a cold and against transmission to another person.
 
‘But it gives you very strong protection against hospitalisation and death.’


Associate Professor Paul Griffin says despite increasing overall number of fully-vaccinated people contracting COVID-19, the rate of infection in those that are vaccinated will remain lower than in those that aren’t. 

Professor Booy’s assessment has been backed by studies in countries where the rate of infection has been much higher than in Australia. In Israel, for example, Pfizer was cited by the country’s Ministry of Health as 93% effective in preventing hospitalisation, despite a lower efficacy in preventing symptomatic infection against the Delta strain.
 
Meanwhile, research published in The New England Journal of Medicine suggests that both Pfizer and AstraZeneca have high efficacy rates after two doses even against emerging variants, although with much more limited impact after a single dose.
 
Similar views on vaccine efficacy were expressed by a range of infectious disease and health experts approached for their insights into COVID-19 infections among fully vaccinated people.
 
Associate Professor Paul Griffin, an infectious diseases physician at the University of Queensland, makes the point that as more people are vaccinated, the number of cases within that vaccinated population will also rise.
 
‘Unless a vaccine prevents transmission 100% 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,’ he said.
 
‘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.’
 
Associate Professor Griffin also notes that in other parts of the world, the greater freedoms given to vaccinated people may be influencing the infection rate.
 
‘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,’ he said.
 
‘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.’
 
Professor Nikolai Petrovsky is at Flinders University’s College of Medicine and Public Health, and also works as a research director at South Australian-based biotechnology company Vaxine, which is currently developing a COVID-19 vaccine.
 
He says that while vaccines are not as effective against more recent virus variants as they are against the original strain, they still have an important part to play.
 
‘[Vaccines] clearly do play a major role in reducing hospitalisations and death, just as seasonal influenza vaccines do,’ Professor Petrovsky said. 
 
‘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.’
 
Transmissibility
Professor Booy says that, as well as personal protection, vaccination helps contribute towards herd immunity.
 
‘People who get infected tend to have much lower viral load and of course don’t get severe disease,’ he said. ‘And because they have lower viral load in their throat, they cough and breathe out the virus less often – there’s less virus in your throat to expel.’
 
The exact level of the viral load caused by the Delta strain among the fully vaccinated is still unclear.


Professor Nikolai Petrovsky says while current COVID-19 vaccines are not perfect, they play a major role in reducing hospitalisations and death.
 
In its latest Science Brief, the US Centers for Disease Control and Prevention (CDC) suggests the number of breakthrough infections caused by the Delta virus is higher than in other strains, and that those who are fully vaccinated could pass it on.
 
This week it also updated its advice for fully vaccinated people to wear a mask in public indoor settings in areas of high transmission.
 
‘These early data suggest that breakthrough Delta infections are transmissible,’ the report states.
 
‘Unpublished data are consistent with this, and additional data collection and studies are underway to understand the level and duration of transmissibility from Delta vaccine breakthrough infections in the United States and other settings.’
 
The CDC reiterates the broad protection offered by the vaccines against severe illness, but is clear that full vaccination should not be viewed as a complete safeguard for all.
 
‘In general, symptoms and duration of illness in vaccine breakthrough infections have been attenuated compared with cases among unvaccinated people,’ the update says.
 
‘Among hospitalised or fatal breakthrough cases reported to CDC as of July 19, 2021, 74% were aged 65 years or older.
 
‘One US study observed that 44% of breakthrough infections were among people who were immunocompromised, similar to results reported from Israel.’
 
The report also suggests potential good news for fully vaccinated people who subsequently test positive for COVID-19 and have only mild symptoms.
 
‘Breakthrough infections may boost immunity; four weeks after an outbreak in a long-term care facility, fully vaccinated residents who experienced breakthrough infections were found to have significantly higher antibody levels than vaccinated individuals who did not experience breakthrough infections,’ the authors say.
 
The right message
Clear messaging is – as with so much of pandemic management – likely to be key to ensuring those who are vaccinated are aware of the level of protection the vaccines offer, a point also made by Professor Petrovsky.
 
‘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,’ he said.
 
Professor Booy, meanwhile, believes that GPs are in an ideal position to guide people through what the vaccines should be expected to achieve.
 
‘We need to have clear explanation to patients to manage their expectations – and GPs are well trusted by their patients and suited to doing that,’ he said.
 
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