As the country begins to see the effect of the 4th wave, only about a third of the eligible population in South Africa have been fully vaccinated. Vaccination prevents infection and reduces the severity of coronavirus disease 2019 (COVID-19), while unvaccinated individuals are at higher risk and continue to make up the majority of cases involving hospitalizations and fatalities.
A growing literature of studies show a clear reduction in positivity rates where there is high vaccination uptake, which is hardly surprising given the consistent findings demonstrating significant COVID vaccine efficacy against disease transmission.
This too was seen in a recent study investigating whether vaccination could reduce transmission in the household setting in the context of post-vaccination infection.
Data from the Household Transmission Evaluation Dataset (HOSTED), which comprises information on all laboratory-confirmed cases of COVID-19 was used. This was linked to data on all persons sharing the same address and was coupled to individual-level data on all COVID-19 vaccinations in England.
The study compared the risk of secondary infection (defined as a positive SARS-CoV-2 test - 2 to 14 days after the positive test for the index case) among unvaccinated household contacts of persons with SARS-CoV-2 infection who had received at least one dose of the ChAdOx1 nCoV-19 or BNT162b2 vaccine 21 days or more before testing positive with the risk among unvaccinated household contacts of unvaccinated persons with infection.
Significance of findings
Between 4th January and 28th February 2021, there were 960,765 household contacts of unvaccinated index patients, and there were 96,898 secondary cases of Covid-19, which is approximately 10.1% compared to 5.7% and 6.2% for those who received the ChAdOx1 nCoV-19 or BNT162b2 vaccines, respectively.
Most of the vaccinated index patients in this data set (93%) had received only the first dose of the vaccine, with similar results obtained for both vaccines.
The likelihood of household transmission was reduced by 40 - 50% in households of index patients who had been vaccinated 21 days or more before testing positive than in households of unvaccinated index patients. Assessment of infection risks among household contacts according to the timing of vaccination of the index patient showed protective effects when the vaccine had been administered at least 14 days before the positive test.
No-one is safe until everyone is safe – this is why we need a global response to COVID-19.
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