Maximum protection from COVID-19 is achieved after two doses of COVID-19 vaccine. Everyone aged from 12 years is recommended to received two doses of COVID-19 vaccine because when more people are vaccinated, they are less likely to be infected, and if they are infected, will expel less virus for a shorter time. Vaccinated people are at a much lower risk of becoming seriously unwell if they catch COVID-19. The Delta variant is highly infectious and so we need a high level of vaccine coverage to slow its spread and in addition to the current social distancing and mask wearing. Reducing the spread will lower the risk that new more vaccine-resistant variants will emerge.

One dose provides some protection in the short term, particularly against severe disease, but a second dose from at least 3 weeks (not sooner) after dose one to ensure more immediate and longer lasting protection against Delta variant.

Is the Delta variant more able to spread?

  • Higher viral load – compared with people infected with the original strain, preprint data show that the virus was detected within 4 days of exposure compared with 6 days average for the original strain, and those infected have viral loads up to 1,260 times higher than those infected with the original strain.(1)
  • Spreads for longer before symptoms appear - a study in China found that people had symptoms 1.8 days on average after their first positive COVID-19 test for viral mRNA and 5.8 days after signs of infection. This provided nearly 2 days to shed virus before they showed any signs of COVID-19. In comparison, prior to the emergence of Delta, COVID-19 infections took an estimated average of 6.3 days to develop symptoms and 5.5 days to test positive, leaving only 0.8 days of unknown shedding. Around three-quarters of infections with Delta in close contacts occurred during the presymptomatic phase, higher than for previous variants.(2)
  • The odds of household transmission for Delta is almost two-thirds more than for the Alpha variant.(3, 4)

What makes Delta more transmissible?

Mutations help the Delta variant to enter the human cells more readily and to spread between lung cells. The spike protein of the SARS-CoV-2 Delta variant can bind to the ACE2 receptor more strongly than other variants and enables the virus to evade the immune system.

  • The SARS-CoV-2 virus hijacks the cell’s machinery during its replication phases to focus production on making new viral particles.
  • Alternations in the spike protein allow an enzyme called furin to prime it to fuse more readily with the cell membrane, allowing the virus to enter the cell directly without alerting the immune system.
  • In the SARS-CoV virus (that caused the SARS outbreak in early 2000s), fewer than 10% of the spike proteins are primed. For the original SARS-CoV-2 virus, this rose to almost half (50%). Over 50% were primed for the Alpha variant, and for the Delta strain, more than 75% of the spikes are primed to infect human cells.

See this Nature news feature article for more detailed information on the adaptations.

Is the Delta variant more harmful?

A Public Health England analysis has indicated that a greater proportion of Delta cases need emergency care than those with the Alpha variant.

Patients in the UK infected with Delta were shown to be more than twice as likely to be hospitalised or need emergency care within 14 days of a positive COVID-19 test than those infected with Alpha variant (hazard ratio 2.26 [95% CI 1.32 to 3.89]). Three-quarters of the patients were unvaccinated and the median age was 31 years.(5)

How well does vaccination with the Comirnaty mRNA vaccine protect against Delta?

Maximum effectiveness against COVID-19 is achieved by at least 7 days after two doses of the vaccine. Current evidence shows that the vaccine remains protective against COVID-19, notably against severe disease.

Two doses of mRNA vaccine, Comirnaty, has been shown to be effective against symptomatic COVID-19 caused by Delta (78-93% compared with 90-96% for Alpha variant) in the UK. However, after only one dose vaccines are less effective against Delta than Alpha (21-44% vs 47-55%, combined for Comirnaty and COVID-19 vaccine AstraZeneca). These data shown the importance of being fully vaccinated with two doses.(6) Further data from England has shown that one dose does provide excellent protection against severe disease: after at least 21 days, vaccination is 46-99% effective against hospitalisation due to the Delta variant but just 25-36% effective against all symptomatic disease, including milder disease. Two doses (given 12 weeks apart) provide excellent protection against both hospitalisation (86-99%) and all symptomatic disease (92-95%) at least 14 days after vaccination(non-peer-reviewed, study funded by from Public Health England).(7)
A study in Norway also found that two doses of COVID-19 vaccine was needed to be best protected against COVID-19 Delta variant. Fully vaccinated individuals were two-thirds less likely to be infected with Delta compared with unvaccinated individuals [hazard ratio 0.35 [0.32-0.39]).(8)


  1. Li B, Deng A, Li K, Hu Y, Li Z, Xiong Q, et al. Viral infection and transmission in a large, well-traced outbreak caused by the SARS-CoV-2 Delta variant. medRxiv. 2021 (preprint):2021.07.07.21260122.
  2. Xin H, Wong JY, Murphy C, Yeung A, Taslim Ali S, Wu P, et al. The Incubation Period Distribution of Coronavirus Disease 2019: A Systematic Review and Meta-analysis. Clinical Infectious Diseases. 2021;corrected proof.
  3. Public Health England. SARS-CoV-2 variants of concern and variants under investigation in England. Technical Briefing 15. Public Health England; 2021 11 June 2021.Available from:…
  4. Allen H, Vusirikala A, Flannagan J, Twohig K, Zaidi A, COG-UK Consortium, et al. Increased household transmission of COVID-19 cases associated with SARS-CoV-2 Variant of Concern B.1.617.2: a national case-control study2021 (preprint). Available from:…
  5. Twohig KA, Nyberg T, Zaidi A, Thelwall S, Sinnathamby MA, Aliabadi S, et al. Hospital admission and emergency care attendance risk for SARS-CoV-2 delta (B.1.617.2) compared with alpha (B.1.1.7) variants of concern: a cohort study. The Lancet Infectious Diseases.
  6. Lopez Bernal J, Andrews N, Gower C, Gallagher E, Simmons R, Thelwall S, et al. Effectiveness of Covid-19 Vaccines against the B.1.617.2 (Delta) Variant. N Engl J Med. 2021.
  7. Stowe J, Andrews N, Gower C, Gallagher E, Utsi L, al e. Effectiveness of COVID-19 vaccines against hospital admission with the Delta (B.1.617.2) variant. 2021 (Public Health England preprint).
  8. Seppälä E, Veneti L, Starrfelt J, et al. Vaccine effectiveness against infection with the Delta (B.1.617.2) variant, Norway, April to August 2021. Euro Surveillance. 2021;26(35). doi: 10.2807/1560-7917.ES.2021.26.35.2100793
Last updated: 18 November 2021