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expert reaction to observational study on transmission of COVID-19 in households of vaccinated healthcare workers

Public Health Scotland (PHS) have published an observational study on the transmission of COVID-19 in households of vaccinated healthcare workers.


Prof Deborah Dunn-Walters, Chair of the British Society for Immunology COVID-19 and Immunology Taskforce and Professor of Immunology at the University of Surrey, said:

“Understanding whether vaccines prevent transmission as well as stopping disease is a crucial piece of the puzzle in building our knowledge of the potential impact of vaccines on disease spread within our communities. This new preprint from Public Health Scotland on a large cohort of healthcare workers shows some of the first evidence that COVID vaccines can reduce transmission of the virus between individuals as well as preventing people getting very ill with disease. Importantly, these effects appear to be measurable after one dose of the vaccine and increase after the second dose.  This is extremely promising news. 

“While this study brings welcome news, it is still early days and there is still much we need to learn about how COVID vaccines affect transmission. For example, this study was carried out on healthcare workers, but we also need to understand whether the same effects are experienced by older people after vaccination.  For now, it’s very important for us all to keep following current social distancing guidelines even if we’ve been vaccinated.” 


Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice Magazine, Immediate past Chair of the BMA Public Health Medicine Committee, said:

“A year ago there were concerns that we might be unable to create vaccines that would be effective against Coronaviruses such as SARS-CoV-2, the virus causing Covid-19 disease; that such vaccines would be very limited in effectiveness, in the duration of effectiveness, or that they would prevent disease but not prevent infection and transmission.

“We have been pleasantly surprised by the emerging evidence: that protection from vaccination exceeds protection following natural infection; that vaccines are effective at preventing death, serious disease, hospital admission, mild to moderate and even asymptomatic infection.

“When the phase III studies appeared, there were strong suggestions – from the type of immunity they induced – that vaccines may be able to prevent infection and transmission. Since the vaccines have been used, studies have reported that they prevent – at least to some extent – asymptomatic infection.

“This study, however, is the first one that I have seen that shows direct evidence that vaccination can reduce transmission.

“Healthcare workers (HCWs) are at increased risk of Covid-19 infection – although, being of working age (“18-65 years of age”), they are largely at relatively low risk of serious disease. Many, if infected, will have asymptomatic infection; and they will transmit the infection to their household contacts. Of course, there are other potential sources of infection for HCWs’ household contacts, so even if the HCW in the household were unable to transmit the infection, there would still be some cases in the household. Nevertheless, a previous study (discussed in this paper) showed that household members of HCWs were at double the risk of Covid-19 infection compared to the general population.

“This study shows a reduction in Covid-19 infection rates in the household contacts of HCWs who have been vaccinated (compared to household contacts of HCWs who have not been vaccinated). As the press release says: “Where healthcare workers had received a second dose of the vaccine at least 14 days before, their household members had a rate of Covid-19 which was at least 54% lower than household members where healthcare workers had not been vaccinated.”

“This reinforces our hopes that vaccination will be able to reduce the effective R number (Rt), and thus to reduce transmission. Whether vaccination alone will be sufficient – eventually, when enough people have been vaccinated – to bring Rt below one, and keep it there – so we achieve “population-” or “herd immunity” is not yet clear; but the findings of this study give us increased hope that it might.”

Does the press release accurately reflect the science?


Is this good quality research?  Are the conclusions backed up by solid data?

“Yes, this is excellent research, using population data, with many data points, allowing firm conclusions with reasonably narrow confidence intervals.

How does this work fit with the existing evidence? Is this the first real world study in the UK looking at transmission?

“As the authors point out, the study fits with various other studies which looked at infection (including in some cases asymptomatic infection) rates, not just disease rates, in people who have been vaccinated.2-5 It also fits with previous studies which have showed the sort of immunity that one might expect to reduce or prevent infection and transmission. As far as I am aware, however, this is the first study to report direct evidence of reduced disease in contacts of people who have been vaccinated.

Have the authors accounted for confounders?  Are there important limitations to be aware of?

“The statistical analysis appears robust and to have made appropriate allowances.

As the authors point out, all the HCWs vaccinated were of working age, so the findings do not necessarily apply to younger or older people.

The vast majority of healthcare workers in this study received the Pfizer-BioNTech vaccine (referred to in the paper as “BNT162b2 mRNA”). A minority received the Oxford/AstraZeneca vaccine (referred to as “ChAdOx1 nCoV-19”). The data do not allow robust assessment of the relative effectiveness of the two vaccines.

What are the implications in the real world?  Is there any overspeculation?

“As above. There is no overspeculation.

Any other comments.

“The paper expresses its findings as Hazard ratios, or HR. It doesn’t say what HR stands for in the abstract, which may lead to confusion. There is, for example, a sentence in the press release which reads:

“Where healthcare workers had received a second dose of the vaccine at least 14 days before, their household members had a rate of Covid-19 which was at least 54% lower than household members where healthcare workers had not been vaccinated.”

“The corresponding sentence in the abstract reads:

“Compared to the period before the first dose, the risk of documented COVID-19 case was lower at ≥14 days after the second dose for household members (HR 0∙46 [95% CI 0∙30to 0∙70]) and healthcare workers (HR 0∙08 [95% CI 0∙04 to 0∙17]).”

“The 54% referred to in the press release relates to the “HR 0.46”. The hazard ratio the rate in household members of vaccinated HCWs was 0.46 of the rate in household members of non-vaccinated HCWs, which is a reduction of 1-0.46 or 54%.”

  1. Shah ASV, Gribben C, Bishop J, Hanlon P, Caldwell D, Wood R, et al. Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households. 2021; Updated; Accessed: (
  2. Dagan N, Barda N, Kepten E, Miron O, Perchik S, Katz MA, et al. BNT162b2 mRNA Covid-19 vaccine in a nationwide mass vaccination setting. N Engl J Med 2021. (
  3. Chodick G, Tene L, Patalon T, Gazit S, Tov AB, Cohen D, et al. The effectiveness of the first dose of BNT162b2 vaccine in reducing SARS-CoV-2 infection 13-24 days after immunization: real-world evidence. medRxiv 2021:2021.01.27.21250612. ( or
  4. Vasileiou E, Simpson CR, Robertson C, Shi T, Kerr S, Agrawal U, et al. Effectiveness of First Dose of COVID-19 Vaccines Against Hospital Admissions in Scotland: National Prospective Cohort Study of 5.4 Million People [pre-print]. 2021; Updated 19 Feb 2021; Accessed: 2021 (01 Mar): (
  5. Hall V, Foulkes S, Saei A, Andrews N, Oguti B, Charlett A, et al. Effectiveness of BNT162b2 mRNA vaccine against infection and COVID-19 vaccine 1 coverage in healthcare workers in England, multicentre prospective cohort study (the 2 SIREN study) [submitted manuscript]. The Lancet 2021; Updated 22 Feb 2021; Accessed: 2021 (05 Mar): (


Prof Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham, said:

“This study finds that the household members of vaccinated healthcare workers are 30% less likely to be infected. The main issue is that this study underestimates the degree of protection as household contacts of vaccinated workers can acquire infection from other sources.  This means that some of the cases in this group could never be prevented by vaccine of a household member leading to an underestimate of the degree of secondary protection.

“It is not surprising that vaccination prevents transmission as we have increasing evidence the vaccines can stop disease and asymptomatic infection and you can only spread an infection if you have that infection.

“The effect on transmission after one dose further supports the UK’s decision to use a 12 week interval to maximise the numbers vaccinated despite the criticism from many despite no evidence for their assertions.

“As cases continue to decline, when there is a localised increase in cases the policy of considering vaccinating younger age groups in that area needs to be considered and modelled.”


‘Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households’ by Anoop Shah et al. is a preprint, available here:


All our previous output on this subject can be seen at this weblink:


Declared interests

Prof Deborah Dunn-Walters: No conflicts

Dr Peter English: No conflicts

None others received.


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