The Delta variant is behaving very differently than other COVID-19 variants. It causes different symptoms and is more transmissible. We must remain vigilant about the risks.

The Delta variant poses a serious risk to the health of vulnerable groups, especially in those who are not yet fully vaccinated. It currently accounts for 91% of new cases in the UK, 40% in the US and is driving cases across Europe. 

Studies show that the AstraZeneca and Pfizer vaccines are still effective against the Delta variant. While their effectiveness is reduced at preventing symptomatic disease, they still appear to remain highly effective at preventing hospitalisation and death. This is likely to be true for other vaccines too.

What is the Delta variant?

The Delta variant was responsible for the significant loss of life in India during the April and May peak and has since been identified in the UK, US, Canada, China and continental Europe, among others. It contains mutations first identified in the Alpha and Beta variant, as well as potentially novel mutations.

The Delta variant is the most prevalent member of the B.1.617 group of variants, with the related Kappa variant also reported as a variant of concern. Some reports also currently refer to a “Delta Plus” variant, which has an added mutation, but these have yet to be formally validated and recognised by the World Health Organization.

Is transmissibility impacted?

Initially suspected to have elevated transmissibility based on the alarming increase in cases in India, the Delta variant is now estimated to have an R0 potentially as high as 6, with 60% increased transmissibility over the Alpha. The Alpha variant itself was predicted to be 40 – 60% more transmissible than the original virus in Wuhan, indicating a substantially increased overall transmissibility of this variant.

The Delta variant now makes up 91% of all new cases in the UK, having out-competed the Alpha variety. Delta is also driving case numbers across continental Europe, notably in Spain, Portugal and France, leading to some social distancing measures being reapplied. Current reports from the US suggest that just under half of all COVID-19 cases are caused by the Delta variant, which is likely to become dominant by the middle of July.

How are the symptoms different?

The well-known COVID-19 symptoms of the altered sense of taste and smell, a new sustained cough and fever do not seem to be as common with the Delta variant. Research from the Zoe study in the UK suggests that headaches, muscle aches, sneezing, sore throat and runny noses are far more common which could easily be mistaken for a heavy cold or mild flu.

This is especially the case in the vaccinated or young and healthy populations. People with these symptoms may not suspect that they have COVID-19 and may be less likely to isolate and get tested. This could compound the already high transmissibility of the variant as patients become symptomatic spreaders.

Are vaccines still working?

Preliminary research from England has indicated that the Delta variant remains susceptible to vaccines, but with slightly reduced effectiveness when compared with the native virus. Pfizer (96%) and AstraZeneca (92%) vaccines were shown to be highly effective after both doses at preventing hospitalisation. Recent evidence from Israel indicates that Pfizer is less effective at preventing symptomatic COVID-19 and has led to a reintroduction of social distancing measures in the country.

The study shows a 93% effectiveness against hospitalisation, but only a 63% effectiveness against the disease itself. If true, this is a substantial decline in effectiveness which could be a cautionary warning for hospital capacities and countries looking to reopen. Given the similar mechanism, it can also be argued that Moderna would behave similarly to Pfizer, and Sputnik and Johnson & Johnson would behave similarly to AstraZeneca.

Vaccine efficacy for Delta is seen to be highly dependent on full vaccination status. Those hospitalised in England due to the Delta variant had almost exclusively not had both doses of vaccines. Evidence for a vaccine delay of 8 – 12 weeks from the first to the second dose appeared to be beneficial in strengthening the vaccine agents the native virus or Alpha variant.

However, this appears to be reversed for the Delta variant, with the recommendation now being to prioritise second doses where possible for enhanced protection. Israel vaccinated almost exclusively with Pfizer and with a small gap between doses at the start of the year. This may indicate that the effectiveness of the vaccines has started to decline over time (approximately six months) or that the shortened interval between doses also limited the length of protection.