To judge from recent scientific and media output, there appear to be two parallel realities currently existing side-by-side in Covid world. In one, the vaccines are highly effective at preventing infection and transmission, and any data that suggests otherwise is being misrepresented or is biased or contains some kind of basic error.
In the other – the one that bears a much closer resemblance to the one we actually live in – vaccine effectiveness against infection has been declining significantly and after six months is basically zero. At some point, one of these realities is going to have to give way because they can’t both be true. I know which one my money’s on.
An example of the first appeared in New Scientist this week, headlined: “How much less likely are you to spread COVID-19 if you’re vaccinated?” The answer: at least 63%, according to a new population-based pre-print study from the Netherlands:
A recent study found that vaccinated people infected with the Delta variant are 63% less likely to infect people who are unvaccinated.
This is only slightly lower than with the Alpha variant, says Brechje de Gier at the National Institute for Public Health and the Environment in the Netherlands, who led the study. Her team had previously found that vaccinated people infected with Alpha were 73% less likely to infect unvaccinated people.
What is important to realise, de Gier says, is that the full effect of vaccines on reducing transmission is even higher than 63%, because most vaccinated people don’t become infected in the first place.
De Gier and her team used data from the Netherlands’ contact tracing system to work out the so-called secondary attack rate – the proportion of contacts infected by positive cases. They then worked out how much this was reduced by vaccination, adjusting for factors such as age.
The data comes from August and September 2021, when Delta was dominant in the Netherlands. The key table, breaking the figures down by whether the index case and contacts were vaccinated, via link below.
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