UK COVID Stats - Credit PHE
In countries - including the United States - where substantial numbers of people have been vaccinated, COVID cases, hospitalizations, and deaths have decreased (see charts above and below) strongly suggesting that they are effective in reducing the impact of the pandemic.
The question over just how effective these vaccines are, and when benefits can be expected on a population level, is less easily quantified. To that end, this week the UK has released a pre-print study on the effectiveness of 1 dose of either the Pfizer-BioNTech or AstraZeneca vaccine in reducing household transmission of the virus.
While 1 dose of either of these COVID vaccines is believed less than optimal, in some regions of the world the decision has been made to delay the second dose in order to get more people vaccinated. Understanding how much protection this first dose gives in reducing transmission is therefore important.
We've a press release from the UK's PHE, a link to some expert reaction on the UK Science Media Centre on their findings, followed by a link to the study. First stop, this press release from the PHE.
One dose of COVID-19 vaccine can cut household transmission by up to half
A new study by Public Health England (PHE) has shown that one dose of the COVID-19 vaccine reduces household transmission by up to half.From:Public Health England Published:28 April 2021
Studies have already demonstrated that being vaccinated against coronavirus (COVID-19) significantly reduces your risk of being infected.
This new research shows that those who do become infected 3 weeks after receiving one dose of the Pfizer-BioNTech or AstraZeneca vaccine were between 38% and 49% less likely to pass the virus on to their household contacts than those who were unvaccinated.
Protection was seen from around 14 days after vaccination, with similar levels of protection regardless of age of cases or contacts.
This protection is on top of the reduced risk of a vaccinated person developing symptomatic infection in the first place, which is around 60 to 65% – 4 weeks after one dose of either vaccine.
Households are high-risk settings for transmission and provide early evidence on the impact of vaccines in preventing onward transmission. Similar results could be expected in other settings with similar transmission risks, such as shared accommodations and prisons.
By linking case and household contact data with vaccination status, the study compared the likelihood of transmission for a vaccinated case with an unvaccinated one.
The study included over 57,000 contacts from 24,000 households in which there was a lab-confirmed case that had received a vaccination, compared with nearly 1 million contacts of unvaccinated cases.
Health and Social Care Secretary, Matt Hancock, said:
This is terrific news – we already know vaccines save lives and this study is the most comprehensive real-world data showing they also cut transmission of this deadly virus.It further reinforces that vaccines are the best way out of this pandemic as they protect you and they may prevent you from unknowingly infecting someone in your household.I urge everybody to get their vaccines as soon as they are eligible and make sure you get your second dose for the strongest possible protection.This is a huge national effort and we will beat the virus together.
Dr Mary Ramsay, Head of Immunisation at PHE, said:
Vaccines are vital in helping us return to a normal way of life. Not only do vaccines reduce the severity of illness and prevent hundreds of deaths every day, we now see they also have an additional impact on reducing the chance of passing COVID-19 on to others. I encourage anyone who is offered a vaccine to take it as soon as possible.While these findings are very encouraging, even if you have been vaccinated, it is really important that you continue to act like you have the virus, practise good hand hygiene and follow social distancing guidance.
Previous PHE studies have shown that both the Pfizer-BioNTech and AstraZeneca vaccines are highly effective in reducing COVID-19 infections among older people, with 10,400 deaths prevented in the over-60s by the end of March.PHE is also undertaking separate studies on the effect of vaccination on transmission in the wider population.
The reassuring tone of the press release is echoed by two expert reactions posted yesterday on the SMC (Science Media Centre) website (excerpts below).
APRIL 28, 2021
Expert reaction to preprint looking at COVID-19 vaccination and household transmission
A preprint, an unpublished non-peer reviewed study, by Public Health England (PHE) suggests one dose of either the Oxford-AstraZeneca or Pfizer-BioNTech COVID-19 vaccine may reduce household transmission by up to half.
Prof Deborah Dunn-Walters, Chair of the British Society for Immunology COVID-19 Taskforce and Professor of Immunology at the University of Surrey, said:
“To understand the full impact on vaccines on disease spread, we need to know if they prevent transmission as well as stopping vaccinated individuals from getting sick with COVID. This new preprint from Public Health England on over 550,000 households provides further evidence that the Pfizer/BioNTech and Oxford/AstraZeneca COVID vaccines are effective in reducing transmission of the virus between individuals as well as preventing people getting very ill with disease. This is very promising.
“While this study brings welcome news, we must not be complacent. There is still much we need to learn about how COVID vaccines affect transmission. It is still very important for us all to get two doses of the COVID vaccine to ensure we receive the optimal and longest lasting protection, both for ourselves and our communities.”
Below you'll find a link to the pre-print, and an excerpt from its conclusion. Follow the link to read the 31-page PDF in its entirety.
‘Impact of vaccination on household transmission of SARS-COV-2 in England
by Harris et al was under embargo until 00:01 UK time on Wednesday 28th April 2021.
Conclusions
In addition to the direct effects of preventing cases and reducing severity, we have shown that both the ChAdOx1 nCoV-19 and BNT162b2 vaccines are associated with reduced likelihood of household transmission by 40-50% from individuals diagnosed with COVID-19 after vaccination, highlighting important wider benefits to close contacts.
While this analysis was primarily intended to understand impacts on transmission to household contacts rather than those outside the household, the former are consistently identified as being at high risk for secondary infection. Therefore, these results could also have implications for transmissibility in other settings with similar transmission risks. These would need to be considered in future pandemic modelling to fully capture the impact of the vaccination programme and to inform public health strategies and public communication going forward.
While there are limitations to this study (as there are to all studies), these results are encouraging. There are, of course, open questions about how long vaccine protection will last, and how well they will deal with emerging variants, but for now the evidence continues to suggests that COVID vaccines are doing pretty much what we hoped they would do.
