How dangerous is Delta?

The delta variant of SARS coronavirus-2 first appeared in India in December 2020 and within three months became the predominant variant across the country. By May 2021, thousands of people were dying daily in India from infections with suspected this variant. It also spread rapidly in England in the second quarter of this year. The World Health Organization (WHO) lists the delta variant as one of the four current SARS-CoV-2 variants of concern, and it has been circulating in Germany since early May, accounting for nearly half of all positive tests by the end of June.  

The delta variant shows changes at several sites in the spike protein, the characteristic surface protein of SARS-CoV-2. The virus uses the spike protein to dock with human cells in order to infect them. Researchers are trying to find out what effects the mutations have in various projects. Do they affect the transmissibility of the virus?  How well does the immune system recognize the delta variant after an infection or vaccination?

Rapid spread, higher infection, low death rate

The fact that the delta variant spreads so quickly may also have something to do with the symptoms it triggers: Headaches now top the list of recorded covid symptoms, followed by a runny nose. Fewer patients, on the other hand, report loss of taste and shortness of breath. Because they do not recognize the corona infection as such, they may unknowingly infect people around them.

Public Health England (PHE) evaluated results from nearly 400,000 infections with alpha and delta variants confirmed by sequencing or genotyping for the first half of 2021. Patient data for all of April and May 2021 showed that those infected with the delta variant were about one-third more likely to pass on the virus than those infected with the alpha variant. Particularly within a household, the risk of infection appears to be higher for those infected with the delta variant than for those infected with the alpha variant. Meanwhile, almost all postitive test samples sequenced in England contain the delta variant.

An analysis of data from the EAVE II (Early Pandemic Evaluation and Enhanced Surveillance of COVID-19) study at the University of Edinburgh in mid-June showed that patients infected with the delta variant were hospitalized significantly more often than those infected with alpha. This trend was confirmed, albeit less pronounced, over several weeks in England. According to a PHE study, the death rate among patients infected with the delta variant has remained consistently low at 0.2% since its emergence in late March 2021. Protection from severe and fatal courses by vaccination likely plays an important role.

How do virus mutations occur?

• Unlike bacteria, viruses need a host cell to survive. They penetrate the host cell with their genetic information and use its cell function to reproduce. The genetic information, the genome, is copied. During infection, the virus multiplies its genetic material millions of times.
• Small errors can occur during this process, most of which have no consequences. Some of the random changes that occur can influence the fitness of the virus: For example, it may replicate faster, remain outside the body longer, or be more poorly recognized by the immune system.
• Coronaviruses have mechanisms that enable them to correct errors in the transcription of the genetic material, which is why they mutate less than some other viruses.
• Nevertheless, the higher the level of infection, i.e., the more people are infected at any given time, the more mutations can occur. As long as SARS-CoV-2 is in circulation, new variants will continue to emerge - always with the risk that they will make the virus more dangerous for humans.
• Therefore, an important goal of vaccination is not only to protect individuals from becoming ill but also to protect the population from the spread of the virus.

Full vaccination protects

The Scottish and English data also show that full vaccination protects well against symptomatic illness and against hospitalization for infection with the delta variant of the virus. In May, the finding that the first dose of vaccination alone provides significantly less protection against infection with the delta variant had attracted much attention. However, with the second dose of vaccine, this difference is virtually evened out.

In this regard, the vaccine from BioNTech/Pfizer more reliably prevents infection with the delta variant and symptomatic disease than that from AstraZeneca. The various studies on this are based on observational data from healthcare systems and multiple statistical methods and should always be interpreted with some caution. In Germany, there have also been some small studies suggesting that cross-vaccination with AstraZeneca followed by mRNA vaccine provides better protection against coronavirus variants than dual vaccination with AstraZeneca. How safely and for how long prior SARS-CoV-2 infection protects against infection with a new variant or against disease remains unclear.

In Germany, the Robert Koch Institute (RKI) is pooling data collected by public health departments, virus sequences and laboratory tests on the occurrence of variants.

Since the end of June, delta has also been dominant here, with about three-quarters of all positive samples analyzed containing this viral variant at the beginning of July. This figure approximates the proportion of those infected with delta among all newly registered cases. In Germany, too, a larger proportion of patients infected with the delta variant were initially hospitalized, according to reported data. By early July, the proportion of hospital admissions among patients infected with the delta and alpha variants converged to about one in 20 confirmed cases for both.

Protecting the younger ones

The spread of the apparently more contagious delta variant has also fueled public debate about vaccination for children and adolescents. In England, where delta is now the predominant variant, it appears that children and adolescents are more likely to be infected than people aged 25 and older. Yet there have been no more hospital admissions in the five-to-24 age groups than among older people. In Germany, according to the RKI, the proportion of patients infected with delta who have been admitted to hospital is slightly higher in each of the age groups between five and 34 than that infected with alpha. Here, the summer school vacations could slow down the spread of the virus among students. Nevertheless, some experts warn of an increasing shift of infections to the unvaccinated age groups. A significant proportion of those infected could suffer long-term consequences - with potentially serious societal consequences. They urge maintaining non-pharmaceutical interventions, such as wearing masks and spacing out, to protect young people. For schools, they recommend appropriate ventilation and hybrid teaching models of present and online adjunct. In addition, they advise considering universal vaccination recommendations for children and adolescents. Vaccinating parents and contacts of students as fully as possible will also help keep schools running.

The COVID-19 pandemic poses immense challenges to our society and to each and every one of us. As Germany's largest research organization, Helmholtz is making important contributions to address the corona crisis through cutting-edge research.

Background information and facts can be found on our topic page on the coronavirus