Thread by @CillianDeGascun

There’s been a lot of discussion recently about #SARSCoV2 #vaccine effectiveness, hospitalisation, and the reason for increasing case numbers, in spite of our high rates of vaccination. This thread will not provide a definitive explanation for the above 1/n

However, I’m hopeful it will provide context to inform the discussion. TL:DR The vaccines are not 100% effective so our current level of uptake – measured across the entire population – will not be sufficient, in isolation, to suppress #SARSCoV2 transmission in the community 2/n

Over the last 18 months, the concept of “herd immunity” has been frequently referenced in public discourse: I prefer, and will use, the term “community level protection (CLP)”, thereby illustrating the concept that immunising the community is capable of 3/n

protecting the individual who, for whatever reason, might not be able to receive the vaccine, or the individual whose immune system might not respond to the vaccine. Taking this concept of CLP, there is a number of elements that we need to take into account 4/n

First, the reproductive number (R0) of the pathogen In question (i.e. #SARSCoV2); second, the effectiveness of the available vaccine(s); third, the vaccine uptake rate in the population; & fourth, the crude CLP threshold, which can be derived from the R0 of the pathogen 5/n

Before discussing #SARSCoV2 however, and to illustrate the significance of the above parameters, I am first going to discuss 4 other viruses: #smallpox, #rubella, #measles, and #mumps 6/n

1: Firstly, with an R0 of 5-7, a CLP threshold (CLPT) of 80-85%, a vaccine effectiveness (VE) of 95%, and vaccine uptake of >80% (at least, globally), #smallpox has now been eradicated 7/n

2: Similarly, with an R0 of 6-7, a CLPT of 83-85%, a VE of 99-100% (single dose), and vaccine uptake of >92% (in Ireland, @hpscIreland data), endemic rubella has been eliminated from Ireland 8/n

3: Likewise, with an R0 of 12-18, a CLPT of 92-94%, a VE of 95% (single dose), & vaccine uptake of >92% (@hpscireland data), endemic measles has been eliminated from Ireland (Of note, the VE of 95% against measles virus necessitated the second dose of the MMR vaccine) 9/n

4: With an R0 of 4-7, CLPT of 75-85%, VE of 65-75%, and vaccine uptake of >92% (@hpscireland data), endemic mumps has not been eliminated in Ireland (pre-pandemic, at least). This is almost entirely as a result of the reduced VE of the mumps component of the MMR vaccine 10/n

If we apply the above parameters to #SARSCoV2, we have an R0 of 5-8, a CLPT of 80-88%, a VE of 75% (range 53-91%, per SY Tartof et al, Lancet 2021), and vaccine uptake of 88% in those more than 12 years of age 11/n

However, this level of vaccine uptake – when measured across the entire population – equates to somewhere between 75% and 80%. On that basis, and taking into account the cohort of the population that is not (yet, at least) eligible for vaccination, we can see 12/n

that the current first generation of #SARSCoV2 vaccines is unlikely – in isolation at least, i.e. without additional public health measures – to be able to control #SARSCoV2 Delta in the community 13/n

So, to simplify the above (& with apologies to the actual modellers & statisticians in the audience), vaccine uptake rate multiplied by vaccine effectiveness should be greater than the CLPT for a virus to be brought under control 14/n

In the case of #SARSCoV2, this threshold has not been reached with the current vaccines. As a result, we will continue to see case numbers increase as restrictions ease. This does not, and I stress, does not, mean that the #SARSCoV2 vaccines are not effective 15/n

It simply means that they are not sufficiently effective – on their own – to suppress or eliminate #SARSCoV2 transmission in the community. However, they are very effective at protecting against hospitalisation and severe disease 16/n

PS: for this thread, I took R0 & CLP data from the 7th edition of Stanley Plotkin’s #Vaccines. Any errors or misinterpretations are my own 17/17

PPS: whilst vaccination reduces the risk of Delta infection, fully vaccinated individuals have viral loads similar to unvaccinated & can efficiently transmit infection in household settings, including to fully vaccinated contacts (Singanayagam et al, medRxiv, Oct 2021) 18/18