The UK, following other countries, is now giving a 3rd dose of vaccine. Whilst the Pfizer, AstraZeneca and other vaccines protect against severe infection, they only are able to reduce, but not eliminate, transmission. Despite the success of the vaccine roll out, increasing numbers of fully (ie double) vaccinated people are becoming infected with the Delta variant. What was unknown until research just published this week is just how much protection do these vaccines provide against the transmission of Delta? This new research also addresses two additional questions: (i) are there any differences between the Pfizer and AstraZeneca vaccines and (ii) by how much does the ability to protect against transmission wane over time?
How can vaccines protect against transmission?
- In thinking about the protection from the vaccines, we need to distinguish between the following two scenarios:
- As you will see, these are not the same question and the answers are different (read on!)
- Vaccines may work in several ways to stop both types of transmission
- Amongst the theories are:
- They can stop some people getting the infection at all, by attacking the virus before it can take hold
- They don’t stop people becoming infected (ie they will still have a positive test), but stop people getting symptoms and for example coughing out the virus
- If people become infected despite being vaccinated, the amount of virus they shed (‘viral load’) is lower
- If people become infected despite being vaccinated, the amount of time they shed virus is lower
These are the questions for which up to now the answers are unclear
- Vaccines stop us getting very ill, but by how much do they stop the virus being transmitted?
- If I have been double vaccinated, by how much will that stop me picking up the infection from being in contact with another infected person?
- Will my risk be different if I had the Pfizer or the AstraZeneca vaccine?
- How much will that risk wane over the months since I have been vaccinated?
- There is a separate question which relates to the risk that unvaccinated people pose to me.
- Thus if I am in contact with an infected person, how much less is my risk of getting infected if that person had been vaccinated?
- Will that risk be different if they had the Pfizer or the AstraZeneca vaccine?
- How much will that risk wane over the months since they had been vaccinated
Sorry I don’t mean to make this seem complicated but it is important to be very precise about the question being addressed. Hopefully the results below will help if you are confused! Feel free to skip down to my conclusions at the end of this post
The Oxford/NHS England Research
- The research comes from linking the NHS Test and Trace system with the national vaccine database and has emerged in a non peer-reviewed paper
- This is what the researchers did
- The NHS ‘Test and Trace’ system was used to identify all new cases – called an ‘index case’ (ie the person was not previously known to the system) – who had tested positive from a PCR test
- The system (like many other national systems) then attempted to follow up any known contacts of the infected person
- Those contacts who agreed then had a PCR test
- The research also identified through the national vaccine database the double vaccination status of:
- the index case
- the contacts of the index case
- They separately identified from the vaccine database which vaccine had been used (AstraZeneca or Pfizer – the data was gathered before Moderna was introduced) and what was the interval between the second dose of the vaccine and the PCR test
What did they find: the benefits of vaccine to the contacts?
- I have expressed the results comparing the risk of becoming infected when contacts had been vaccinated compared to those who had not (which I fixed at 100%).
- The results at first sight look impressive:
- If a contact had the Pfizer vaccine they would have just 10% of the risk of catching an infection compared to the contacts who had not been vaccinated
- If a contact had the AstraZeneca vaccine they would have 28% of the risk of catching an infection compared to contacts who had not been vaccinated
- The protection from Pfizer was thus noticeably higher than from AstraZeneca
- This protection contacts had from their vaccines becomes much less strong, the longer the interval since their second vaccine dose. This is true for both Pfizer and AstraZeneca
- Let me try and explain the graph below
- The dotted line “unvaccinated contact” gives the proportion of contacts who when tested had a positive PCR. This was around 0.7 (or 70%).
- This proportion seems very high, but the Test and Trace system typically follows up on close – mainly household – contacts.
- It is also possible that those contacts who were closest and/ or feeling unwell were more likely to turn up for testing
- The red line is the experience of those who had the AstraZeneca vaccine
- This showed that the proportion of contacts tested with a positive PCR increases, the longer the interval since they had received their second dose
- Thus by 14 weeks after the second AstraZeneca dose about 0.5 (50%) of contacts tested were positive on PCR
- The blue line is the experience of those who had the Pfizer vaccine
- This showed that the proportion of contacts tested with a positive PCR increases, the longer the interval since they had received their second dose.
- Thus by 14 weeks after the second Pfizer dose about 0.4 (40%) of contacts tested were positive on PCR
- What do these results mean?
- The biggest challenge in interpreting these results is that we only know about the contacts who came for testing
- Those contacts that were tested were more likely to have symptoms and to have been in closer contact with the index case, hence the positive rates will be higher
- Nonetheless the headline conclusion is that:
- Vaccination gives a moderate degree of protection against getting infected when you are in close contact with an infected person
- This protection is higher following Pfizer than AstraZeneca vaccine
- The protection does wane quite quickly
What did they find about transmission from a vaccinated person?
- For this part of the research they focused their analysis on those who had been vaccinated and (despite their vaccine) still became infected
- They were compared with those who had not been vaccinated and became infected
- They wanted to see if the close contacts of vaccinated people had a lower risk of being positive than the close contacts of people who were unvaccinated
- This is what they found:
- Again, for comparison purposes I have set the risk of an unvaccinated person passing the infection on to contacts as 100 (%)
- As you can see:
- if the infected person had had the Pfizer jab, they were had around 35% of the risk of that from unvaccinated people to pass on the infection to a contact
- if the infected person had had the AstraZeneca jab, they were had around 64% of the risk of that from unvaccinated people to pass on the infection to a contact
- Maybe these results are not surprising, ie once a vaccinated person becomes infected then why wouldn’t they be able to pass the infection on to close contacts?
- The research also looked at how time since vaccination affected their risk of passing the infection on
- This is what they found:
- In some ways these data are similar to the analysis undertaken from the perspective of the vaccine status of the contact (see above).
- The bottom line is that if a vaccinated person gets an infection, then by 14 weeks after their second dose their risk of passing the infection to a close contact is almost the same as if they had not been vaccinated
How reliable are these data?
- Being honest, these are very difficult questions to address and difficult studies to do
- The risks of any of us becoming infected depends on so many things – just look at the list below:
- The researchers attempted to address all these issues and try and identify the specific contribution of vaccine status
- But there are always remaining concerns that the results may not be that accurate – although these inaccuracies should be the same for both Pfizer and AstraZeneca
- We already knew that vaccines did not stop transmission and that doubly vaccinated people could still contract the infection from another infected person
- That this risk declines with time since vaccination is also not too much of a surprise, though the speed of the decline, in these data, is perhaps unexpected
- It also seemed reasonable that if, despite being vaccinated, a person contracted the infection, they would still be able to pass this on to others
- We might have hoped that the vaccination might have lowered the amount of virus they shed and reduced the risk to contacts, but this was not the case
- Whilst AstraZeneca vaccine has enormous benefits in cutting down on serious illness, we don’t know why it is less impressive than Pfizer in stopping transmission
- The key question though is what do these data tell us about the role of boosters in cutting transmission? To be honest I am really not sure
- It is hoped that boosters might make it harder to transmit the virus and may increase the duration of protection against transmission. We just don’t know yet
- Worth repeating at the end of this post – these data do NOT change what we know about the success of the vaccines at stopping serious disease which boosters should enhance
One final thought about compulsory vaccination as a public health message!
There is much heated debate, and worry, that our being exposed to people who are unvaccinated puts us at greater risk. These new data do not argue against the benefits of vaccination but perhaps limit by how much our own protection is enhanced. To me I would rather the person sitting opposite to me on the train today had worn a mask (which as I wrote this he was not*) than was vaccinated!
*He did lend me his iPhone charger though!
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