{"id":3560,"date":"2022-02-04T15:39:59","date_gmt":"2022-02-04T15:39:59","guid":{"rendered":"http:\/\/wp.cov19longhaulfoundation.org\/?p=3560"},"modified":"2022-02-04T15:39:59","modified_gmt":"2022-02-04T15:39:59","slug":"what-do-we-know-about-covid-vaccines-and-preventing-transmission","status":"publish","type":"post","link":"https:\/\/cov19longhaulfoundation.org\/?p=3560","title":{"rendered":"What do we know about covid vaccines and preventing transmission?"},"content":{"rendered":"\n

BMJ<\/em> 2022; 376 doi: https:\/\/doi.org\/10.1136\/bmj.o298<\/a>  (Published 04 February 2022) Cite as:  BMJ<\/em> 2022;376:o298<\/p>\n\n\n\n

Authors: Chris Stokel-Walker,<\/p>\n\n\n\n

Vaccines that work against SARS-CoV-2 have helped change the course of the pandemic by reducing illness and hospital admissions. But Chris Stokel-Walker<\/strong> asks what we know about their impact on preventing transmission.<\/p>\n\n\n\n

The range of vaccines developed in record time by pharmaceutical companies and research laboratories have helped quell the worst effects of SARS-CoV-2. But much of the focus of research has been on effectiveness in preventing infection, illness, and hospital admission. What is less well measured is the impact of vaccination on preventing onward transmission.<\/p>\n\n\n\n

What evidence do we have that covid-19 vaccines prevent transmission?<\/h2>\n\n\n\n

Most papers to date (notably, many are preprints and have yet to be peer reviewed) indicate vaccines are holding up against admission to hospital and mortality, says Linda Bauld, professor of public health at the University of Edinburgh, \u201cbut not so much against transmission.\u201d<\/p>\n\n\n\n

The first weekly covid-19 vaccine surveillance report for 20221<\/a> from the UK Health Security Agency (UKHSA) was more positive than Bauld\u2019s assessment\u2014but didn\u2019t say outright that covid-19 vaccines prevent transmission. \u201cSeveral studies have provided evidence that vaccines are effective at preventing infection,\u201d it states, \u201cUninfected people cannot transmit; therefore, the vaccines are also effective at preventing transmission.\u201d<\/p>\n\n\n\n

A study2<\/a> of covid-19 transmission within English households using data gathered in early 2021 found that even a single dose of a covid-19 vaccine reduced the likelihood of household transmission by 40-50%. This was supported by a study of household transmission among Scottish healthcare workers conducted between December 2020 and March 2021.3<\/a> Both studies analysed the impact of vaccination on transmission of the \u03b1 variant of SARS-CoV-2, which was dominant at the time.<\/p>\n\n\n\n

A subsequent study,4<\/a> conducted later in the course of the pandemic when the delta variant was dominant, showed vaccines had a less pronounced effect on denting onward transmission, but were still effective.<\/p>\n\n\n\n

How could vaccines help reduce transmission?<\/h2>\n\n\n\n

Vaccines aren\u2019t preventing onward transmission by reducing the viral load\u2014or amount of SARS-CoV-2\u2014in your body. \u201cMost studies show if you got an infection after vaccination, compared with someone who got an infection without a vaccine, you were pretty much shedding roughly the same amount of virus,\u201d says Paul Hunter, professor in medicine at the University of East Anglia. One study,5<\/a> sponsored by the US Centers for Disease Control and Prevention (CDC), found \u201cno difference in infectious virus titer between groups\u201d who had been vaccinated and had not.<\/p>\n\n\n\n

Instead, it\u2019s the principle that the UKHSA identified above: if you don\u2019t get infected in the first place thanks to a vaccine, you can\u2019t spread it. Once you\u2019re infected, you still can\u2014although what we know about the window when you\u2019re most likely to transmit the virus to others has improved.<\/p>\n\n\n\n

Does the omicron variant make a difference?<\/h2>\n\n\n\n

Few studies have looked at the omicron variant, although a report published in January 2022 by the European Centre for Disease Prevention and Control cited a small Danish household study:6<\/a> \u201cPeople who have completed the primary series of vaccination experienced secondary attack rates (SARs) of 32% in households with omicron and 19% in households with delta. For people who received a booster, omicron was associated with a SAR of 25%, while the corresponding estimate for delta was only 11%. There was an increased transmission for unvaccinated people, and a reduced transmission for booster vaccinated people, compared with fully vaccinated people,\u201d summarised the report.7<\/a><\/p>\n\n\n\n

Preliminary data from Japan\u2019s National Institute of Infectious Diseases found that patients infected with omicron shed viral particles for longer compared with those infected with other variants. The amount of viral RNA in patients with omicron was highest three to six days after diagnosis or symptom onset. This appears to be two or three days later than other variants.8<\/a> Hunter said the new data \u201cmuddy the waters\u201d on the matter.<\/p>\n\n\n\n

Vaccine effectiveness against infection, hospital admission, and mortality have all taken a hit when pitted against the omicron variant, and it seems only logical that the impact against transmission would likewise drop.<\/p>\n\n\n\n

\u201cThe main point of vaccines is not to do with preventing transmission,\u201d says Anika Singanayagam, academic clinical lecturer in adult infectious disease at Imperial College London. \u201cThe main reasons for vaccines for covid-19 is to prevent illness and death.\u201d Therefore, we shouldn\u2019t be too disappointed that it\u2019s still possible to pass on the virus while vaccinated, she says, \u201cDamping down on transmission is not a particularly easy thing with omicron.\u201d<\/p>\n\n\n\n

What impact does that have on policymaking?<\/h2>\n\n\n\n

The fact that vaccines are good at preventing serious infection, but less good at preventing transmission makes policymaking difficult. The UK has changed its rules9<\/a> on the amount of time those who test positive for covid-19 must spend in self-isolation, first from 10 days to seven, then to five, provided they test negative on a lateral flow test. That decision follows the US, which cut the self-isolation period to five days in late December10<\/a> because \u201cthe majority of SARS-CoV-2 transmission occurs early in the course of illness.\u201d<\/p>\n\n\n\n

\u201cThey\u2019re recognising that vaccines aren\u2019t preventing transmission, and you\u2019ve got too many people having to isolate,\u201d says Bauld. \u201cPolicymakers have decided that the game\u2019s up on transmission, but that you need a different approach.\u201d<\/p>\n\n\n\n

Decision makers have a difficult decision, says Singanayagam: they want to enable life to continue as normally as possible\u2014which may mean vaccinated people getting infected with covid because of community or household transmission\u2014while also carefully monitoring that vaccine effectiveness to lower the risk of hospital admission, severe illness, and death is not dented.<\/p>\n\n\n\n

Could future vaccines be more effective against onward transmission?<\/h2>\n\n\n\n

Again, first generation covid vaccines were evaluated against reducing hospital admissions and death in the challenging first year of the pandemic. They wouldn\u2019t have been expected to generate sterilising immunity and block transmission. But, says Singanayagam, now that we have a suite of vaccines using different approaches, there is some opportunity to think about future jabs for different situations.<\/p>\n\n\n\n

\u201cThere are avenues to think about the development of vaccines that can have more of an effect on transmission,\u201d she says. Those are usually vaccines delivered more locally, such as directly through the respiratory tract, which could tackle the source of major transmission, rather than the lungs, which is where the first generation of vaccines was targeted in order to prevent severe infection. \u201cThat\u2019s probably the way things will move in the future.\u201d<\/p>\n\n\n\n

This article is made freely available for personal use in accordance with BMJ’s website terms and conditions for the duration of the covid-19 pandemic or until otherwise determined by BMJ. You may download and print the article for any lawful, non-commercial purpose (including text and data mining) provided that all copyright notices and trade marks are retained. https:\/\/bmj.com\/coronavirus\/usage<\/a><\/p>\n\n\n\n

References<\/h2>\n\n\n\n
  1. \u21b5<\/a>UK Health Security Agency. Covid-19 vaccine surveillance report: week 1. 6 January 2022. https:\/\/assets.publishing.service.gov.uk\/government\/uploads\/system\/uploads\/attachment_data\/file\/1045329\/Vaccine_surveillance_report_week_1_2022.pdf<\/a><\/li>
  2. Dabrera G.Dunbar JK,Andrews NJ, Zaidi A, Hall JA, Harris RJ, Effect of vaccination on household transmission of SARS-CoV-2 in England. N Engl J Med2021;385:759-60.  doi:10.1056\/NEJMc2107717  pmid:34161702CrossRef<\/a>PubMed<\/a>Google Scholar<\/a><\/li>
  3. \u21b5<\/a>Shah ASV, Gribben C, Bishop J, et al. Effect of vaccination on transmission of covid-19: SARS-Cov-2. N Engl J Med2021;385:1718-20. doi:10.1056\/NEJMc2106757 pmid:34496200 CrossRef<\/a> PubMed<\/a> Google Scholar<\/a><\/li>
  4. \u21b5<\/a>Eyre DW, Taylor D, Purver M, et al. The impact of SARS-CoV-2 vaccination on alpha and delta variant transmission.Medrxiv2021.09.28.21264260 [Preprint]. doi:10.1101\/2021.09.28.21264260CrossRef<\/a> Google Scholar<\/a><\/li>
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  6. \u21b5<\/a>Lyngse FP, Mortensen LH, Denwood MJ, et al. SARS-CoV-2 omicron VOC transmission in Danish households.Medrxiv2021.12.27.21268278 [Preprint]. doi:10.1101\/2021.12.27.21268278CrossRef<\/a> Google Scholar<\/a><\/li>
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  8. \u21b5<\/a>Torjesen I. Covid-19: Peak of viral shedding is later with omicron variant, Japanese data suggest. BMJ2022;376:o89. doi:10.1136\/bmj.o89 pmid:35027360FREE Full Text<\/a> Google Scholar<\/a><\/li>
  9. \u21b5<\/a>Mayor S. Covid-19: Warning over transmission risk as self-isolation is cut to five days in England. BMJ2022;376:o111. doi:10.1136\/bmj.o111 pmid:35031534FREE Full Text<\/a>Google Scholar<\/a><\/li>
  10. \u21b5<\/a>Centers for Disease Control and Prevention. CDC updates and shortens recommended isolation and quarantine period for general population. 27 December 2021. www.cdc.gov\/media\/releases\/2021\/s1227-isolation-quarantine-guidance.html<\/a><\/li><\/ol>\n\n\n\n

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