What do we know about covid vaccines and preventing transmission?

BMJ 2022; 376 doi: https://doi.org/10.1136/bmj.o298  (Published 04 February 2022) Cite as:  BMJ 2022;376:o298

Authors: Chris Stokel-Walker,

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 asks what we know about their impact on preventing transmission.

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.

What evidence do we have that covid-19 vaccines prevent transmission?

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, “but not so much against transmission.”

The first weekly covid-19 vaccine surveillance report for 20221 from the UK Health Security Agency (UKHSA) was more positive than Bauld’s assessment—but didn’t say outright that covid-19 vaccines prevent transmission. “Several studies have provided evidence that vaccines are effective at preventing infection,” it states, “Uninfected people cannot transmit; therefore, the vaccines are also effective at preventing transmission.”

A study2 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 Both studies analysed the impact of vaccination on transmission of the α variant of SARS-CoV-2, which was dominant at the time.

A subsequent study,4 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.

How could vaccines help reduce transmission?

Vaccines aren’t preventing onward transmission by reducing the viral load—or amount of SARS-CoV-2—in your body. “Most 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,” says Paul Hunter, professor in medicine at the University of East Anglia. One study,5 sponsored by the US Centers for Disease Control and Prevention (CDC), found “no difference in infectious virus titer between groups” who had been vaccinated and had not.

Instead, it’s the principle that the UKHSA identified above: if you don’t get infected in the first place thanks to a vaccine, you can’t spread it. Once you’re infected, you still can—although what we know about the window when you’re most likely to transmit the virus to others has improved.

Does the omicron variant make a difference?

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 “People 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,” summarised the report.7

Preliminary data from Japan’s 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 Hunter said the new data “muddy the waters” on the matter.

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.

“The main point of vaccines is not to do with preventing transmission,” says Anika Singanayagam, academic clinical lecturer in adult infectious disease at Imperial College London. “The main reasons for vaccines for covid-19 is to prevent illness and death.” Therefore, we shouldn’t be too disappointed that it’s still possible to pass on the virus while vaccinated, she says, “Damping down on transmission is not a particularly easy thing with omicron.”

What impact does that have on policymaking?

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 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 because “the majority of SARS-CoV-2 transmission occurs early in the course of illness.”

“They’re recognising that vaccines aren’t preventing transmission, and you’ve got too many people having to isolate,” says Bauld. “Policymakers have decided that the game’s up on transmission, but that you need a different approach.”

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

Could future vaccines be more effective against onward transmission?

Again, first generation covid vaccines were evaluated against reducing hospital admissions and death in the challenging first year of the pandemic. They wouldn’t 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.

“There are avenues to think about the development of vaccines that can have more of an effect on transmission,” 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. “That’s probably the way things will move in the future.”

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


  1. 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
  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:34161702CrossRefPubMedGoogle Scholar
  3. 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 PubMed Google Scholar
  4. 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 Google Scholar
  5. Riemersma KK, Grogan BE, Kita-Yarbro A, et al. Shedding of infection SARS-CoV-2 despite vaccination.Medrxiv2021.07.31.21261387 [Preprint]. doi:10.1101/2021.07.31.21261387Cross RefGoogle Scholar
  6. 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 Google Scholar
  7. European Centre for Disease Prevention and Control. Assessment of the further spread and potential impact of the SARS-CoV-2 omicron variant of concern in the EU/EEA, 19th update. 27 January 2022. www.ecdc.europa.eu/en/publications-data/covid-19-omicron-risk-assessment-further-emergence-and-potential-impact
  8. 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 Google Scholar
  9. 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 TextGoogle Scholar
  10. 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

FDA Issues Warning About Increased Risk Of Heart Inflammation Caused By Moderna Jab

Authors: BY TYLER DURDENMONDAY, AUG 30, 2021 – 02:14 P

Earlier this month, we reported on leaked data from a Canadian study which arrived at a disturbing conclusion: the risk of rare side effects like myocarditis and pericarditis – types of heart inflammation that are potentially deadly in some patients – was at least 2.5x higher in the Moderna jab than in its main competitor, produced by Pfizer-BioNTech.

The leaking of the data to the press was an embarrassment for the FDA and CDC, and so they pledged to investigate. Now, less than two weeks later, the FDA has just announced that it has updated its “fact sheet” to reflect the higher risk of heart inflammation in male patients under the age of 40.

For all patients, the “post-marketing” data examined by the FDA show that the risk of experiencing these side effects is highest within 7 days of receiving the second dose.

Only Pfizer has received full approval from the FDA; the Moderna jab is still technically under the emergency authorization. Whether this will delay or in any way impact the FDA’s approval of the Moderna jab remains unclear.

Here’s the full updated text:

Myocarditis and Pericarditis Postmarketing data demonstrate increased risks of myocarditis and pericarditis, particularly within 7 days following the second dose. The observed risk is higher among males under 40 years of age than among females and older males. The observed risk is highest in males 18 through 24 years of age. Although some cases required intensive care support, available data from short-term follow-up suggest that most individuals have had resolution of symptoms with conservative management. Information is not yet available about potential long-term sequelae. The CDC has published considerations related to myocarditis and pericarditis after vaccination, including for vaccination of individuals with a history of myocarditis or pericarditis

Questions about the link between the mRNA jabs and heart inflammation have been circulating since these side effects were first uncovered in a group of American soldiers reporting acute chest pain after their vaccinations.

The news is weighing on Moderna’s share price, which has fallen substantially since its Aug. 9 peak. It was down more than 3% on Monday afternoon.

Reactogenicity Following Receipt of mRNA-Based COVID-19 Vaccines

Authors: Johanna Chapin-Bardales, PhD, MPH1Julianne Gee, MPH1Tanya Myers, PhD, MSc1

In December 2020, 2 mRNA-based COVID-19 vaccines (Pfizer-BioNTech and Moderna) were granted Emergency Use Authorization by the US Food and Drug Administration as 2-dose series and recommended for use by the Advisory Committee on Immunization Practices.13 In late February 2021, the US Food and Drug Administration granted Emergency Use Authorization for a third COVID-19 vaccine, a single-dose adenovirus vector-based vaccine from Janssen (Johnson & Johnson).

In clinical trials of the mRNA-based 2-dose vaccines, participants reported local and systemic reactions (reactogenicity).4,5 Frequently reported reactions included injection site pain, fatigue, and headache; greater reactogenicity was reported following the second dose.4,5 Continued monitoring of reactogenicity of COVID-19 vaccines outside of clinical trial settings may provide additional information for health care practitioners and the public about transient local and systemic reactions following COVID-19 vaccination.

V-safe Active Surveillance System

To facilitate rapid assessment of COVID-19 vaccines, in 2020, the Centers for Disease Control and Prevention (CDC) established v-safe, a new active surveillance system for collecting near–real-time data from COVID-19 vaccine recipients in the US. V-safe participants voluntarily self-enroll and receive periodic smartphone text messages to initiate web-based health surveys from the day of vaccination (day 0) through 12 months after the final dose of a COVID-19 vaccine.6 From day 0 through day 7 after each vaccine dose, participants are asked questions about solicited local and systemic reactions (eg, injection site pain, fatigue, headache). These solicited reactions do not include allergic reactions or anaphylaxis; however, v-safe does allow participants to enter free-text information about their postvaccination experience and asks about adverse health events (eg, received medical care). Medically attended events are followed up on through active telephone outreach; future analyses will address these adverse vaccine experiences. This report describes information on solicited local and systemic reactogenicity reported to v-safe on days 0 to 7 after each dose of vaccine from December 14, 2020, through February 28, 2021. Responses were limited to individuals who were vaccinated by February 21, 2021, to allow a 7-day reporting period after the day of vaccination. Preliminary data from v-safe through January 13, 2021, have been previously reported.7 This activity was reviewed by the CDC and was conducted consistent with applicable federal law and CDC policy (see Additional Information).

Self-reported Local and Systemic Reactions Among V-safe Participants

By February 21, 2021, more than 46 million persons received at least 1 dose of an mRNA-based COVID-19 vaccine.8 A total of 3 643 918 persons were enrolled in v-safe and completed at least 1 health survey within 7 days following their first vaccine dose; 1 920 872 v-safe participants reported receiving a second vaccine dose and completed at least 1 daily health survey within 7 days following the second dose. Solicited local and systemic reactions during days 0 to 7 after each dose were assessed.

Most v-safe participants reported an injection site reaction (dose 1: 70.0%; dose 2: 75.2%) or a systemic reaction (dose 1: 50.0%; dose 2: 69.4%) during days 0 to 7 after vaccination (Table). The most frequently reported solicited local and systemic reactions after the first dose of COVID-19 vaccine were injection site pain (67.8%), fatigue (30.9%), headache (25.9%), and myalgia (19.4%). Reactogenicity was substantially greater after the second dose for both vaccines, particularly for systemic reactions, including fatigue (53.9%), headache (46.7%), myalgia (44.0%), chills (31.3%), fever (29.5%), and joint pain (25.6%).Table.  Solicited Local and Systemic Reactionsa to mRNA-Based COVID-19 Vaccines Reported 0 to 7 Days After Vaccination—Centers for Disease Control and Prevention V-safe Surveillance System, December 14, 2020, to February 28, 2021 View LargeDownload

Solicited Local and Systemic Reactionsa to mRNA-Based COVID-19 Vaccines Reported 0 to 7 Days After Vaccination—Centers for Disease Control and Prevention V-safe Surveillance System, December 14, 2020, to February 28, 2021

A greater percentage of participants who received the Moderna vaccine, compared with the Pfizer-BioNTech vaccine, reported reactogenicity; this pattern was more pronounced after the second dose (Table). When stratified by age (<65 vs ≥65 years), differences in reactogenicity by vaccine remained consistent with overall findings (data not shown). Local and systemic reactions were less commonly reported by v-safe participants 65 years and older compared with those younger than 65 years, but greater reactogenicity after the second dose was observed for both age groups (eFigure in the Supplement). For both doses of both vaccines, the percentage of v-safe participants who reported local and systemic reactions was highest on day 1 after vaccination and declined markedly through day 7.

For More Information: https://jamanetwork.com/journals/jama/fullarticle/2778441