COVID-19: About the Delta variant

The SARS-CoV-2 virus that causes COVID-19 has undergone genetic mutations over time as it adapts to humans, leading to the development of new variants of the virus. One of these new variants, the Delta variant, is now the dominant variant globally.

Last updated: 30 September 2021

The Delta variant has spread rapidly worldwide and is now the main variant in most countries. It is the most transmissible variant, spreading a lot more easily than the original version of the COVID-19 virus and other variants.

How Delta is different from earlier variants

Science is telling us the Delta variant has a number of differences compared to earlier variants of the virus. These differences mean that the Delta variant may be a greater threat to the health of individuals who contract the infection and a greater challenge to contain the spread of the virus in an outbreak. For example:

  • delta may cause people to develop more serious COVID-19 illness than other variants of the virus
  • people with a Delta infection may be at higher risk of needing hospital care
  • the chance of infecting others such as within your household or other contacts is very high because Delta is so transmissible. It is estimated that on average, without vaccination, one person infected with Delta may infect 5 or 6 other people. This is how Delta outbreaks in places overseas have grown so rapidly
  • people with Delta infections seem to carry much more virus (have a higher viral load) and for a longer period of time than those infected with the original virus or other variants
  • the time from exposure to the virus until the first positive test is shorter for the Delta variant. On average, a person may be infectious for 1–2 days before they develop symptoms and some people who are infected never develop symptoms (asymptomatic). Asymptomatic people may still be infectious, early on in their infection.

How we can protect ourselves: vaccine effectiveness and health measures

Globally, and here in New Zealand, it is so important to stamp out any community outbreaks as quickly as possible and to ensure very high rates of vaccination.

Being fully vaccinated gives you protection against Delta infection and a very high degree of protection against severe illness, hospitalisation and death. Evidence currently shows:

  • the effectiveness of two doses of the Pfizer vaccine against symptomatic illness is 64–95%
  • the effectiveness of two doses of the Pfizer vaccine against hospitalisation or severe disease due to Delta infection is about 90–96%.

Some countries are reporting that the effectiveness of the vaccine against Delta infection may reduce over time. This is being monitored to help decision-making about whether booster doses of the vaccine may be needed in the future.

Vaccination does help to reduce transmission of the virus. Taking other precautions also remains important in order to continue to protect our communities against Delta. As well as vaccination, early detection of cases and swift contact tracing, and isolation of cases and contacts, is critical.

It will also be important to continue to protect ourselves and our whānau and stop the transmission of the disease by following health habits such as:

  • keep indoor rooms well ventilated (eg, by opening windows and doors) where possible
  • wearing masks and face coverings is one way of keeping yourself safe and protecting others from COVID-19, especially when physical distancing is not possible. Wear masks in confined or crowded environments, such as public transport or supermarkets
  • physical distancing of 2m where possible
  • if you feel unwell or show any symptoms, stay home. Call Healthline and get a COVID-19 test
  • keep a record of where you’ve been or scan in wherever you go using the COVID Tracer app and turn Bluetooth on your phone so you can be contacted if you have been near a case.

Further information on Delta and other variants of interest or concern may be seen on our Science News page

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