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Sunday, November 14, 2021
Effect of Vaccination on Transmission of SARS-CoV-2
https://www.nejm.org/doi/full/10.1056/NEJMc2106757
TO THE EDITOR:
Whether vaccination of individual persons for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) protects members of their households is unclear. We investigated the effect of vaccination of health care workers in Scotland (who were among the earliest groups to be vaccinated worldwide) on the risk of coronavirus disease 2019 (Covid-19) among members of their households.
We evaluated data from 194,362 household members (which represented 92,470 households of 2 to 14 persons per household) of 144,525 health care workers who had been employed during the period from March 2020 through November 2020. The mean ages of the household members and the health care workers were 31 and 44 years, respectively; a majority (>96%) were White. A total of 113,253 health care workers (78.4%) had received at least one dose of either the BNT162b2 (Pfizer–BioNTech) mRNA vaccine or the ChAdOx1 nCoV-19 (Oxford–AstraZeneca) vaccine, and 36,227 (25.1%) had received a second dose.
The primary outcome was any confirmed case of Covid-19 that occurred between December 8, 2020, and March 3, 2021. We also report results for Covid-19–associated hospitalization. The primary time periods we compared were the unvaccinated period before the first dose and the period beginning 14 days after the health care worker received the first dose. No adjustment was made for multiplicity. Events that occurred after any household member was vaccinated were censored. Detailed methods and results, strengths and limitations, and the protocol are provided in the Supplementary Appendix, which is available with the full text of this letter at NEJM.org. This study was approved by the Public Benefit and Privacy Panel (2021-0013), and the scientific officer of the West of Scotland Research Ethics Committee provided written confirmation that formal ethics review was not required.Table 1.
Effect of Vaccination of Health Care Workers on Documented Covid-19 Cases and Hospitalizations in Health Care Workers and Their Households.
Cases of Covid-19 were less common among household members of vaccinated health care workers during the period beginning 14 days after the first dose than during the unvaccinated period before the first dose (event rate per 100 person-years, 9.40 before the first dose and 5.93 beginning 14 days after the first dose). After the health care worker’s second dose, the rate in household members was lower still (2.98 cases per 100 person-years). These differences persisted after fitting extended Cox models that were adjusted for calendar time, geographic region, age, sex, occupational and socioeconomic factors, and underlying conditions. Relative to the period before each health care worker was vaccinated, the hazard ratio for a household member to become infected was 0.70 (95% confidence interval [CI], 0.63 to 0.78) for the period beginning 14 days after the first dose and 0.46 (95% CI, 0.30 to 0.70) for the period beginning 14 days after the second dose (Table 1 and the Supplementary Appendix). Not all the cases of Covid-19 in the household members were transmitted from the health care worker; therefore, the effect of vaccination may be larger.1 For example, if half the cases in the household members were transmitted from the health care worker, a 60% decrease in cases transmitted from health care workers would need to occur to elicit the association we observed (see the Supplementary Appendix).
Cases of Covid-19 were less common among household members of vaccinated health care workers during the period beginning 14 days after the first dose than during the unvaccinated period before the first dose (event rate per 100 person-years, 9.40 before the first dose and 5.93 beginning 14 days after the first dose). After the health care worker’s second dose, the rate in household members was lower still (2.98 cases per 100 person-years). These differences persisted after fitting extended Cox models that were adjusted for calendar time, geographic region, age, sex, occupational and socioeconomic factors, and underlying conditions. Relative to the period before each health care worker was vaccinated, the hazard ratio for a household member to become infected was 0.70 (95% confidence interval [CI], 0.63 to 0.78) for the period beginning 14 days after the first dose and 0.46 (95% CI, 0.30 to 0.70) for the period beginning 14 days after the second dose (Table 1 and the Supplementary Appendix). Not all the cases of Covid-19 in the household members were transmitted from the health care worker; therefore, the effect of vaccination may be larger.1 For example, if half the cases in the household members were transmitted from the health care worker, a 60% decrease in cases transmitted from health care workers would need to occur to elicit the association we observed (see the Supplementary Appendix).
Vaccination was associated with a reduction in both the number of cases and the number of Covid-19–related hospitalizations in health care workers between the unvaccinated period and the period beginning 14 days after the first dose.
Given that vaccination reduces asymptomatic infection with SARS-CoV-2,2,3 it is plausible that vaccination reduces transmission; however, data from clinical trials and observational studies are lacking.4,5 We provide empirical evidence suggesting that vaccination may reduce transmission by showing that vaccination of health care workers is associated with a decrease in documented cases of Covid-19 among members of their households. This finding is reassuring for health care workers and their families.
Anoop S.V. Shah, M.D.
London School of Hygiene and Tropical Medicine, London, United Kingdom
Ciara Gribben, M.Sc.
Jennifer Bishop, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom
Peter Hanlon, M.D.
University of Glasgow, Glasgow, United Kingdom
David Caldwell, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom
Rachael Wood, Ph.D.
University of Edinburgh, Edinburgh, United Kingdom
Martin Reid, B.Sc.
Jim McMenamin, M.D.
David Goldberg, M.D.
Diane Stockton, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom
Sharon Hutchinson, Ph.D.
Glasgow Caledonian University, Glasgow, United Kingdom
Chris Robertson, Ph.D.
University of Strathclyde, Glasgow, United Kingdom
Paul M. McKeigue, Ph.D.
Helen M. Colhoun, Ph.D.
University of Edinburgh, Edinburgh, United Kingdom
David A. McAllister, M.D.
University of Glasgow, Glasgow, United Kingdom
david.mcallister@glasgow.ac.uk
Supported by the British Heart Foundation and Wellcome.
Disclosure forms provided by the authors are available with the full text of this letter at NEJM.org.
This is the New England Journal of Medicine version of record, which includes all Journal editing and enhancements. The Author Accepted Manuscript, which is the author’s version after external peer review and before publication in the Journal, is available under a CC-BY-ND license at PMC8451182. opens in new tab.
This letter was published on September 8, 2021, and updated on October 28, 2021, at NEJM.org.
5 References
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Supplementary Material
Supplementary Appendix PDF 1541KB
Disclosure Forms PDF 402KB
Given that vaccination reduces asymptomatic infection with SARS-CoV-2,2,3 it is plausible that vaccination reduces transmission; however, data from clinical trials and observational studies are lacking.4,5 We provide empirical evidence suggesting that vaccination may reduce transmission by showing that vaccination of health care workers is associated with a decrease in documented cases of Covid-19 among members of their households. This finding is reassuring for health care workers and their families.
Anoop S.V. Shah, M.D.
London School of Hygiene and Tropical Medicine, London, United Kingdom
Ciara Gribben, M.Sc.
Jennifer Bishop, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom
Peter Hanlon, M.D.
University of Glasgow, Glasgow, United Kingdom
David Caldwell, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom
Rachael Wood, Ph.D.
University of Edinburgh, Edinburgh, United Kingdom
Martin Reid, B.Sc.
Jim McMenamin, M.D.
David Goldberg, M.D.
Diane Stockton, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom
Sharon Hutchinson, Ph.D.
Glasgow Caledonian University, Glasgow, United Kingdom
Chris Robertson, Ph.D.
University of Strathclyde, Glasgow, United Kingdom
Paul M. McKeigue, Ph.D.
Helen M. Colhoun, Ph.D.
University of Edinburgh, Edinburgh, United Kingdom
David A. McAllister, M.D.
University of Glasgow, Glasgow, United Kingdom
david.mcallister@glasgow.ac.uk
Supported by the British Heart Foundation and Wellcome.
Disclosure forms provided by the authors are available with the full text of this letter at NEJM.org.
This is the New England Journal of Medicine version of record, which includes all Journal editing and enhancements. The Author Accepted Manuscript, which is the author’s version after external peer review and before publication in the Journal, is available under a CC-BY-ND license at PMC8451182. opens in new tab.
This letter was published on September 8, 2021, and updated on October 28, 2021, at NEJM.org.
5 References
1.
. opens in new tab
. opens in new tab
. opens in new tab. opens in new tab
2.
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3.
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4.
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5.
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Supplementary Material
Supplementary Appendix PDF 1541KB
Disclosure Forms PDF 402KB
How much less likely are you to spread covid-19 if you're vaccinated?
HEALTH
23 October 2021
By Michael Le Page
https://www.newscientist.com/article/2294250-how-much-less-likely-are-you-to-spread-covid-19-if-youre-vaccinated/
People who are fully vaccinated against covid-19 are far less likely to infect others, despite the arrival of the delta variant, several studies show. The findings refute the idea, which has become common in some circles, that vaccines no longer do much to prevent the spread of the coronavirus.
“They absolutely do reduce transmission,” says Christopher Byron Brooke at the University of Illinois at Urbana-Champaign. “Vaccinated people do transmit the virus in some cases, but the data are super crystal-clear that the risk of transmission for a vaccinated individual is much, much lower than for an unvaccinated individual.”
A recent study found that vaccinated people infected with the delta variant are 63 per cent 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 per cent 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 per cent, 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.
De Gier says they cannot calculate the full reduction in transmission due to vaccination, because they don’t know exactly how much vaccination reduces the risk of infection. But even assuming vaccination only halves the risk of infection, this would still imply that vaccines reduce transmission by more than 80 per cent overall.
Others have worked out the full effect. Earlier this year, Ottavia Prunas at Yale University applied two different models to data from Israel, where the Pfizer vaccine was used. Her team’s conclusion was that the overall vaccine effectiveness against transmission was 89 per cent.
However, the data used only went up to 24 March, before delta became dominant. The team is now using more recent data to work out the impact of delta, says Prunas.
The idea that vaccines are no longer that effective against transmission may derive from news reports in July claiming that vaccinated people who become infected “can carry as much virus as others”. Even if this were true, however, vaccines would still greatly reduce transmission by reducing infections in the first place.
In fact, the study that sparked the news reports didn’t measure the number of viruses in someone directly but relied on so-called Ct scores, a measure of viral RNA. However, this RNA can derive from viruses destroyed by the immune system. “You can measure the RNA but it’s rendered useless,” says Timothy Peto at the University of Oxford.
Read more: How mRNA is transforming the way we treat illnesses from flu to cancer
There are now several lines of evidence that Ct scores aren’t a good measure of the amount of virus someone has. Firstly, the fact that infected vaccinated people are much less likely to infect others. Peto has done a similar study to de Gier using contact tracing data from England and gotten similar results.
Secondly, Peto’s team specifically showed that there is little connection between Ct scores and infectiousness. “It appeared people who were positive after vaccination had the same viral load as the unvaccinated. We thought they were just as infectious. But it turns out you are less infectious,” says Peto. “That’s quite important. People were over-pessimistic.”
Yet another line of evidence comes from a study by Brooke. His team took samples from 23 people every day after they first tested positive until the infection cleared and performed tests, including trying to infect cells in a dish with the samples.
With five out of the six fully vaccinated people, none of the samples were infectious, unlike most from unvaccinated people. The study shows that vaccinated people shed fewer viruses and also stop shedding sooner than unvaccinated people, says Brooke.
The one bit of bad news is that Peto’s study shows that the protection a vaccine provides against an infected person infecting others does wane over time, by around a quarter over the three months after a second vaccine dose. “This has made me a believer in boosters,” he says. “They ought to get on with it, given that we are in the middle of a major outbreak [in the UK].”
By Michael Le Page
https://www.newscientist.com/article/2294250-how-much-less-likely-are-you-to-spread-covid-19-if-youre-vaccinated/
People who are fully vaccinated against covid-19 are far less likely to infect others, despite the arrival of the delta variant, several studies show. The findings refute the idea, which has become common in some circles, that vaccines no longer do much to prevent the spread of the coronavirus.
“They absolutely do reduce transmission,” says Christopher Byron Brooke at the University of Illinois at Urbana-Champaign. “Vaccinated people do transmit the virus in some cases, but the data are super crystal-clear that the risk of transmission for a vaccinated individual is much, much lower than for an unvaccinated individual.”
A recent study found that vaccinated people infected with the delta variant are 63 per cent 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 per cent 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 per cent, 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.
De Gier says they cannot calculate the full reduction in transmission due to vaccination, because they don’t know exactly how much vaccination reduces the risk of infection. But even assuming vaccination only halves the risk of infection, this would still imply that vaccines reduce transmission by more than 80 per cent overall.
Others have worked out the full effect. Earlier this year, Ottavia Prunas at Yale University applied two different models to data from Israel, where the Pfizer vaccine was used. Her team’s conclusion was that the overall vaccine effectiveness against transmission was 89 per cent.
However, the data used only went up to 24 March, before delta became dominant. The team is now using more recent data to work out the impact of delta, says Prunas.
The idea that vaccines are no longer that effective against transmission may derive from news reports in July claiming that vaccinated people who become infected “can carry as much virus as others”. Even if this were true, however, vaccines would still greatly reduce transmission by reducing infections in the first place.
In fact, the study that sparked the news reports didn’t measure the number of viruses in someone directly but relied on so-called Ct scores, a measure of viral RNA. However, this RNA can derive from viruses destroyed by the immune system. “You can measure the RNA but it’s rendered useless,” says Timothy Peto at the University of Oxford.
Read more: How mRNA is transforming the way we treat illnesses from flu to cancer
There are now several lines of evidence that Ct scores aren’t a good measure of the amount of virus someone has. Firstly, the fact that infected vaccinated people are much less likely to infect others. Peto has done a similar study to de Gier using contact tracing data from England and gotten similar results.
Secondly, Peto’s team specifically showed that there is little connection between Ct scores and infectiousness. “It appeared people who were positive after vaccination had the same viral load as the unvaccinated. We thought they were just as infectious. But it turns out you are less infectious,” says Peto. “That’s quite important. People were over-pessimistic.”
Yet another line of evidence comes from a study by Brooke. His team took samples from 23 people every day after they first tested positive until the infection cleared and performed tests, including trying to infect cells in a dish with the samples.
With five out of the six fully vaccinated people, none of the samples were infectious, unlike most from unvaccinated people. The study shows that vaccinated people shed fewer viruses and also stop shedding sooner than unvaccinated people, says Brooke.
The one bit of bad news is that Peto’s study shows that the protection a vaccine provides against an infected person infecting others does wane over time, by around a quarter over the three months after a second vaccine dose. “This has made me a believer in boosters,” he says. “They ought to get on with it, given that we are in the middle of a major outbreak [in the UK].”
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