How well do the COVID-19 vaccines protect against COVID-19 Delta variant?

How well do the COVID-19 vaccines protect against COVID-19 Delta variant?

There is emerging evidence showing that existing COVID-19 vaccines effectively protect people against the dominant Delta variant of COVID-19. While the Delta variant is more infectious and associated with higher rates of severe illness requiring hospitalisation, vaccination remains an important and life-saving defence.

Key points:

  • The AstraZeneca, Pfizer, and Moderna vaccines effectively protect against COVID-19 infection, illness, hospitalisation, severe illness, and death due to the Delta variant.
  • Evidence is emerging showing that it is highly likely that the vaccines reduce the risk of transmission and the length of time a person is infectious.
  • Unvaccinated people are significantly more likely to experience symptoms, be sicker, and die from COVID-19, including the Delta variant.
  • People who have only had one dose of a vaccine are more protected from COVID-19, including the Delta variant, than unvaccinated people.
  • People who have received both doses are more protected than people who have received one.
  • The effectiveness of the vaccines reduces over time, but is still at least as effective as the protection afforded by a previous natural infection.
  • Vaccination does not prevent infection and transmission completely, so following public health orders, local social restrictions remain important for vaccinated people.
  • If exposed or infection is suspected, fully vaccinated people should still be tested for COVID-19 and isolate while awaiting results based on local directions.

There have been reports that herd immunity may be challenged by the emergence of the Delta-variant. However, the evidence shows that vaccination remains very strong and important protection for both vaccinated people and unvaccinated people such as children and others who cannot yet receive the vaccine.

Like other vaccines (eg. Influenza), COVID-19 vaccines do not provide 100% immunity. This means that breakthrough infections may occur. Vaccine breakthrough is usually defined as when a vaccinated person gets COVID-19 after at least 14 days of their second dose). If a vaccinated person does become infected though, it doesn’t mean they will get sick.

Vaccines are effective

One study reports that in Singapore,1 any adult with COVID-19 is admitted to hospital even if they do not have symptoms. This means that even people who feel and appear perfectly well are technically counted as hospitalised, which could easily be misinterpreted as evidence showing high hospitalisation rates even among the vaccinated.

In this study, 218 people were identified as having COVID-19 and admitted to hospital. Some (32%) of these people had been vaccinated and of this vaccinated group, 93% had received the Pfizer vaccine. Like Australia, Singapore prioritised vaccinating older adults, so the average median age of those with breakthrough infections was 56 while the non-breakthrough group’s average age was 39.5. Importantly, people who had had both doses were significantly more likely to be asymptomatic, with 28.2 % of fully vaccinated people with breakthrough infections showing no symptoms compared to only 9.2% of the unvaccinated group.1

This highlights why it is important that even vaccinated people get tested for COVID-19 if they think they may have been exposed.

The study also found that unvaccinated people were more likely to experience more symptoms compared to vaccinated people, including; fever (73.9% versus 40.9%), cough (60.8% versus 38%), shortness of breath (13.1% versus 1.4%), sore throat (33.1% unvaccinated versus 25.4%), and diarrhoea (6.2% versus 0%). Interestingly, vaccinated people were more likely to experience a runny nose (23.9% versus 38%); the only symptom that was more common in vaccinated people.

More unvaccinated people than vaccinated people had more severe illness and developed pneumonia (53.1% versus 21.7%), required oxygen (20.8% versus 2.8%) or were admitted to ICU (5.4% versus 0%). Two unvaccinated people required intubation and two died. No vaccinated people required intubation and none died.1

These results support findings from a US study that examined 10,262 breakthrough infections (prior to identified Delta strain emergence in the US) and found that most infections among vaccinated people were mild, 27% were asymptomatic, 10% were hospitalised, and only 2% died.2 In this study, 29 % of the people admitted to hospital and 18% of those who died were asymptomatic or admitted for reasons unrelated to COVID-19.

As we’ve known from the start of the pandemic, even asymptomatic people can infect others. This may be even more serious with the Delta variant because it is more infectious than other forms of the virus.

This is why it is very important that even vaccinated people follow the same public health advice, social restrictions, and health advice as unvaccinated people. Viral load (amount of the virus a person has) is related to infectiousness, and in the Singapore study above, while both vaccinated and unvaccinated people appeared to have similar viral loads at first, viral loads in vaccinated people appeared to decline faster.1

This, as well as other similar studies on vaccine impact on non-Delta variants,3 and wider evidence on vaccine impact on transmission risk reduction,4-7 suggests that vaccines are likely to reduce the risk of transmission due to a quicker reduction in viral load and shortened duration of infectiousness.

In a Scottish study, of 19,543 infections, 377 (1.9%) were admitted to hospital.8 In this study, participants admitted to hospital for reasons unrelated to COVID-19 were included. Of the 19,543, 7,723 people (39.5%) had the Delta strain, of which 134 (1.7%) were admitted to hospital. This study estimated that after 14 days post-second dose, the Pfizer vaccine offered very good protection against infection (92% non-Delta versus 79% Delta) and that the AstraZeneca vaccine was also effective (73% non-Delta versus 60% Delta). Hospitalisation was also more likely among people who had the Delta strain. However, the study could not compare vaccines in terms of hospitalisation.8

These results from Scotland are similar to those of a study from England, including 19,109 cases of COVID-19 with 4,272 Delta variant cases.9 Only modest differences were noted between AstraZeneca and Pfizer, particularly after two doses. This study estimated the effectiveness of vaccines in terms of the development of symptomatic illness. After one dose of AstraZeneca, effectiveness was 30% and 67% after two doses. For Pfizer, effectiveness was 35.6% following one dose and 88% after two doses.

Now that the Moderna vaccine has become available in Australia, examining how effective this vaccine is against the Delta variant is relevant. A comparison of the Pfizer and Moderna vaccines found that while both vaccines were effective for protecting against COVID-19 infection and hospitalisation, effectiveness appeared to be lower against the Delta variant compared to the Alpha variant; Moderna (76%, 95% CI: 58-87%) and Pfizer (42%, 95% CI: 13-62%) among 25,589 vaccinated individuals.10 When comparing rates of infection between matched individuals fully vaccinated with Moderna versus Pfizer across Mayo Clinic Health System sites in Minnesota, Wisconsin, Arizona, Florida, and Iowa, Moderna conferred a two-fold risk reduction against breakthrough infection compared to Pfizer (IRR = 0.50, 95% CI: 0.39-0.64).10

The same study found that the Moderna vaccine was highly effective (real world) against infection (86%, 95% CI: 81-90.6%) and hospitalisation (91.6%, 95% CI: 81-97%) likely to be caused by the Alpha variant.10

In a study from the United Kingdom, the effectiveness of Pfizer, AstraZeneca, and Moderna (one dose only) against the Delta variant was examined based on results from a large, community-based survey of randomly selected households.11

One key result was that vaccination with any of the three reduces the risk of becoming infected with the Delta variant of COVID-19, higher viral load, and experiencing symptoms. For effectiveness against developing a high viral load at 14 days following dose 2, Pfizer was 92% effective (30 days = 90%, 60 days = 85%, 90 days 78%. At 14 days, AstraZeneca was 69% effective and 61% effective after 90 days.

However, as with other studies, the Pfizer and AstraZeneca vaccines were less effective against the Delta variant. At around four to five months following the second dose, both Pfizer and AstraZeneca appear similarly effective. Before this time, Pfizer appears to be more effective for preventing infection. Results also indicated that one dose of the Moderna vaccine was at least as effective as one dose of the two other vaccines. However, this may have been confounded by the generally younger age of Moderna recipients.11 Another important finding was that vaccination is at least as effective as the protection afforded by a previous natural infection. The authors highlight that their findings suggest that achieving as high as possible vaccine coverage is important because unvaccinated people appear to be more vulnerable to infection and poorer outcomes since the vaccines are less effective against the Delta variant.11


While there have been reports that herd immunity may be put at risk by the emergence of new variants that are less susceptible to the existing vaccines, the evidence strongly suggests that each of the three vaccines that are available in Australia are safe and still very effective across important outcomes such as preventing symptomatic illness, hospitalisation, and severe illness.

While it is challenging to measure the impact of vaccines on transmission and death directly, it is highly likely that, based on the evidence, the COVID-19 vaccines are effective for reducing the risk of these outcomes too.

With the Delta- variant now the dominant variety of COVID-19 in Australia and vaccine eligibility criteria broadening to allow more people to receive ‘the jab’ it is more important than ever that people who can receive the vaccine do so as soon as possible.


  1. Chia PY, Xiang Ong SW, Chiew CJ, et al. Virological and serological kinetics of SARS-CoV-2 Delta variant vaccine-breakthrough infections: a multi-center cohort study. medRxiv 2021: 2021.07.28.21261295.
  2. Centers of Diseased Control and Prevention (CDC) COVID-19 Vaccine Breakthrough Case Investigations Team. COVID-19 Vaccine Breakthrough Infections Reported to CDC — United States, January 1–April 30, 2021. MMWR Morb Mortal Wkly Rep 2021; (70): 792-3.
  3. Pritchard E, Matthews PC, Stoesser N, et al. Impact of vaccination on new SARS-CoV-2 infections in the United Kingdom. Nat Med 2021; 27(8): 1370-8.
  4. V Shah AS, Gribben C, Bishop J, et al. Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households. medRxiv 2021: 2021.03.11.21253275.
  5. Harris RJ, Hall JA, Zaidi A, Andrews NJ, Dunbar JK, Dabrera G. Effect of Vaccination on Household Transmission of SARS-CoV-2 in England. New England Journal of Medicine 2021.
  6. Levine-Tiefenbrun M, Yelin I, Katz R, et al. Initial report of decreased SARS-CoV-2 viral load after inoculation with the BNT162b2 vaccine. Nature Medicine 2021; 27(5): 790-2.
  7. Regev-Yochay G, Amit S, Bergwerk M, et al. Decreased infectivity following BNT162b2 vaccination: A prospective cohort study in Israel. The Lancet Regional Health – Europe 2021; 7.
  8. Sheikh A, McMenamin J, Taylor B, Robertson C. SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness. Lancet 2021; 397(10293): 2461-2.
  9. Lopez Bernal J, Andrews N, Gower C, et al. Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant. New England Journal of Medicine 2021; 385(7): 585-94.
  10. Puranik A, Lenehan PJ, Silvert E, et al. Comparison of two highly-effective mRNA vaccines for COVID-19 during periods of Alpha and Delta variant prevalence. medRxiv 2021: 2021.08.06.21261707.
  11. Pouwels KB, Pritchard E, Matthews PC, et al. Impact of Delta on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK. medRxiv 2021: 2021.08.18.21262237.

Micah DJ Peters PhD and Casey Marnie are at the Australian Nursing and Midwifery Federation (ANMF) National Policy Research Unit (Federal Office), and the University of South Australia, Clinical and Health Sciences, Rosemary Bryant AO Research Centre

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