Can a booster following Pfizer and AstraZeneca vaccines stop transmission?

• 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

My conclusions

• 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!