FILE - In this March 29, 2021 file photo, a worker readies syringes with the Moderna COVID-19 vaccine in Metairie, La.

As the COVID-19 pandemic continues across the world, public health officials are watching certain coronavirus mutations and variants that may be more contagious or deadly than the original strain. Viruses constantly change to adapt and survive, and variants emerge when a strain has one or more mutations that differ from others.

The CDC and World Health Organization (WHO) monitor these variations to find out if transmission could lead to a surge in COVID-19 cases and deaths, as well as whether current vaccines can provide protection.

The US classifies them as either a “variant of interest,” which may lead to outbreaks but isn’t widespread in the country; a “variant of concern,” which shows evidence of increased transmission and more severe disease; or a “variant of high consequence,” which makes vaccines and treatments much less likely to work well.

So far, the US hasn’t classified any coronavirus variants as “high consequence,” but numerous strains have been labelled as “variants of concern” that need to be followed closely.

In particular, the Delta variant has drawn focused attention during the past month due to a rapid increase in cases in several countries, including the US.

Here’s what you need to know about the Delta variant

The Delta variant, also known as B1.617.2, can spread more easily, according to the CDC. The strain has mutations on the spike protein that make it easier for it to infect human cells.

That means people may be more contagious if they contract the virus and more easily spread it to others. It is now the dominant strain in the US.

In fact, researchers have said that the Delta variant is about 50 per cent more contagious than the Alpha variant, which was first identified in the UK, according to The Washington Post. Alpha, also known as B.1.1.7, was already 50 per cent more contagious than the original coronavirus first identified in China in 2019.

Public health experts estimate that the average person who gets infected with Delta spreads it to three or four other people, as compared with one or two other people through the original coronavirus strain, according to Yale Medicine.

The Delta variant may also be able to escape protection from vaccines and some COVID-19 treatments, though studies are still ongoing.

The Delta Plus variant, also known as B.1.617.2.1 or AY.1, is considered a “subvariant” of the Delta version, according to CBS News.

It has a mutation that allows the virus to better attack lung cells and potentially escape vaccines.

First identified in India, Delta Plus has now been found in the US, UK, and nearly a dozen other countries. India has labelled it a variant of concern, but up to July 21 the CDC and WHO haven’t.

The Delta variant was first identified in India in December 2020 and led to major outbreaks in the country. It then spread rapidly and is now reported in 104 countries, according to a CDC tracker.

As of early July, Delta has become the dominant form of the coronavirus in the US, UK, Germany, and other countries. In the UK, for instance, the Delta variant now makes up more than 97 per cent of new COVID-19 cases, according to Public Health England.

What are the symptoms of the Delta variant?

The symptoms are similar to those seen with the original coronavirus strain and other variants, including a persistent cough, headache, fever, and sore throat.

At the same time, COVID-19 patients in the UK have reported that some symptoms are slightly different for Delta, according to data from the ZOE COVID Symptom Study. Cough and loss of smell seem to be less common. Headache, sore throat, runny nose, and fever seem to be more common.

Is the Delta variant more deadly?

Scientists are still tracking the data to determine how deadly it is. Based on hospitalisations in the UK, the Delta variant does seem to be more likely to lead to hospitalisation and death, particularly among unvaccinated people, according to a recent study published in The Lancet.

People who haven’t been fully vaccinated against COVID-19 are most at risk, Yale Medicine reported.

In the US, communities with low vaccination rates have seen a jump in cases, particularly in Midwest and Southern states such as Missouri and Arkansas. Outbreaks have also been found in Mountain states, such as Wyoming.

Kids and younger adults who haven’t been vaccinated may be susceptible as well. In the UK, children and unvaccinated adults under age 50 were 2.5 times more likely to become infected with Delta, according to a recent study published by Imperial College London.

What does the Delta variant mean for the vaccinated?

Scientists are looking at how the Delta variant can cause breakthrough cases, or infections among people who are fully vaccinated. So far, they seem to be rare.

In a preliminary analysis, two doses of the Pfizer or Moderna vaccine appeared to be about 88 per cent effective against disease and 96 per cent effective against hospitalisation with the Delta variant, according to Public Health England.

The AstraZeneca vaccine, which has not been authorised for use in the US, was about 60 per cent effective against disease and 93 per cent effective against hospitalisation.

Now vaccine makers are testing booster shots to find out if they can better protect against the Delta variant and other variants that emerge in coming months. Pfizer announced that it will seek FDA authorisation for a booster dose in August, according to CNN.