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Alerts have been issued about the rising number of chickenpox cases in Northern New South Wales this year. Meanwhile, chickenpox continues to spread across Australia with 2,010 notified cases so far this year.

Getting vaccinated can reduce the chance of getting infected. And while you can still get infected if you’re vaccinated, you’re much less likely to become seriously ill.

Here’s what you need to know about chickenpox virus and how it can come back years later as shingles.

What is chickenpox and how does it spread?

Chickenpox is an infection caused by the highly contagious varicella zoster virus. It spreads from respiratory secretions – when people cough, sneeze or talk – and from the skin lesions.

Up to 90% of people who aren’t immune and are in close to someone with chickenpox will also get infected.

Symptoms begin with fever, runny nose, fatigue and cough. A distinctive, fluid-filled blistering rash appears over three to four days.

These symptoms begin two to three weeks after exposure to an infected person.

How likely are you to become seriously ill?

The virus usually causes a mild illness in children but can be severe in adults, and in those with abnormal immune systems such as transplant patients.

It can also affect the unborn fetus if pregnant women are infected, causing a condition called congenital varicella syndrome, with lifelong disability.

One in every 100 people suffers from infection-related complications such as such as secondary skin infections, severe chest infections and brain inflammation. Rarely, the infection can be fatal.

Most people recover without issue. But having a recent chickenpox infection substantially increases the risk of serious bacterial infection such as invasive group A streptococcal infection.

Who is more likely to get it? Can you get it twice?

In Australia, chickenpox tends to occur more often in late winter and early spring, but it can happen any time of year.

Before the vaccine, most cases were in young children. Now adults and teens – especially those over 15 years – are more likely to become seriously unwell and need hospital care.

The virus itself doesn’t change much over time, unlike the flu virus. It’s rare to get chickenpox twice, but it can happen if a person’s immune system becomes weakened, for example by chemotherapy or certain medicines.

What does the virus do in the body?

Once infected, the virus remains dormant or asleep in the body, hiding from the body’s immune system in nerve cells.

Later in life, the virus can reactivate during times of stress, causing shingles (herpes zoster). Older and immunocompromised people are at increased risk.

Shingles causes a painful blistering skin rash. This pain (known as post-herpetic neuralgia) can last even after the rash has gone.

How effective is the chickenpox vaccine?

The varicella vaccine for chickenpox is a live vaccine, meaning it contains a weakened form of the virus that can’t make you sick.

The vaccine is funded by the National Immunisation Program at 18 months, so it’s free. But can be given as early as nine months in an outbreak situation.

A single dose of the varicella vaccine prevents disease in approximately 65% of cases and protects against severe disease in up to 82% of cases. A second dose increases overall protection to about 95%.

A second dose is not currently funded, meaning you’ll have to pay out-of-pocket for it. However, for those aged 14 years and over who aren’t immune, two doses are needed for the best possible protection. This vaccine is usually administered in combination with the measles, mumps and rubella vaccine.

Compared to 2000–2003, rates of hospitalisations from chickenpox declined by more than 74% in 2011–2014. There have been no deaths linked to chickenpox in children under 15 since 2008.

Infections in children too young to be vaccinated have also fallen by 67%. This decline reflects herd immunity, where widespread vaccination reduces the overall circulation of the virus, indirectly protecting those who cannot be immunised.

This is especially important because the varicella vaccine is live and therefore cannot be given to people who are immunocompromised. By ensuring healthy children are vaccinated, we not only protect them individually but also help safeguard vulnerable members of the community from severe disease.

Read more: What is herd immunity and how many people need to be vaccinated to protect a community?

Why don’t we throw ‘chickenpox parties’ anymore?

Chickenpox parties used to intentionally expose adults or children to chickenpox from an infected person, to get the disease. They used to be popular, particularly before introduction of the vaccines.

But while most people will get a mild disease, there’s no way of predicting who will get complications.

Now there’s a vaccine that’s safe and effective, there’s no need to take additional risk attempting to become infected, so avoid chickenpox parties.

How is the shingles vaccine different?

Australia also has a shingles vaccines (Shingrix): a non-live vaccine for adults.

It’s free for Aboriginal and Torres Strait Islander people aged 50 and over, the general population from 65 years onwards, and severely immunocompromised people from 18 years.

This vaccine doesn’t prevent chickenpox, but it boosts the immune system to stop the varicella virus from escaping our nerve cells and causing shingles.

The varicella and shingles vaccine are different and can’t be used in place of one another.

Read more: Shingles vaccination rates rose during the COVID-19 pandemic, but major gaps remain for underserved groups

How can I tell if my kids and I are vaccinated?

The proportion of children who are vaccinated on time is declining. Help keep everyone safe by ensuring your children are up to date with their routine vaccines and get any extra vaccinations recommended before travelling.

The easiest way to check if you or your child has had the chickenpox vaccine is to look at your immunisation record. If it’s not listed, or you can’t find the record, chat with your GP. They can help you check and let you know which vaccine is right for you.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Archana Koirala, University of Sydney and Joel Vosu, University of Sydney

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Archana Koirala has worked on research funded by Australian Government Department of Health, Disability and Ageing and NSW Health. She is the chair of Vaccination Special Interest Group (VACSIG) and a committee member of the Australia and New Zealand Paediatric Infectious Diseases (ANZPID) Network, within Australasian Society of Infectious Diseases (ASID).

Joel Vosu does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.