This is indeed good news – and not unexpected, since the vaccines have been shown to reduce symptoms, and symptomatic disease is what drives transmission.
However, it’s worth being aware that this is the relative risk reduction. The absolute risk reduction (as always) does not look quite so impressive.
In fact, one of the remarkable findings of the study is that of 960,765 unvaccinated household contacts of unvaccinated index cases testing positive, just 10.1% of them (96,898) caught the disease. This means around 90% of unvaccinated people living in the same house as someone with COVID-19 didn’t catch it themselves. This low secondary attack rate is an indication of the level of immunity the population already has to the virus, whether from innate, pre-existing or acquired immunity, even before vaccines come into the picture.
The study identified 3,424 unvaccinated household contacts of index cases who tested positive despite receiving their first dose of the AstraZeneca vaccine at least 21 days earlier. Of these, 196 tested positive themselves, giving a secondary attack rate of 5.7%.
For the Pfizer vaccine the same figures were 371 secondary cases testing positive out of 5,939 unvaccinated household contacts, giving a secondary attack rate of 6.3%.
This means in absolute terms the risk for household members of catching Covid from an infected household index case was reduced from around 10% when the index case was an unvaccinated person to around 6% when he or she was vaccinated, a drop of 4%. This is encouraging, if not as big as might be hoped – though it may improve after the second dose.
For unexplained reasons the study does not look at symptoms at all, so we have no idea how many of the vaccinated positive cases were asymptomatic or how severe their symptoms were. Other studies suggest that positive cases are more likely to be asymptomatic or mild in those with immunity (whether from infection or vaccination) and this is likely to explain much of the drop in secondary attack rate for the household contacts of index cases who are vaccinated but test positive.
This cheering news on the effectiveness of the vaccines for cutting transmission gives the Government even less reason to stick to its glacial reopening strategy.