Australia is heading towards a whooping cough epidemic, with cases surging past 11,000 over the past six months, compared to 2,447 in all of last year.
The highly infectious disease is most dangerous for babies under six months of age, who are too young to be immunised.
In adults, whooping cough can initially feel like another nasty cold or flu, increasing the risk of community transmission.
Previous epidemics have shown new "smarter" strains that can evade vaccines have been evolving.
So how bad could this outbreak get, and what can you do to stay protected and keep the most vulnerable safe?
How to spot whooping cough
Whooping cough, or pertussis, is a severe respiratory disease caused by the bacteria Bordetella pertussis.
It starts with typical cold and flu symptoms, such as a runny nose, sneezing, a mild cough and fever.
So, early on, it's hard to tell it apart from any other nasty cold.
But after about one to two weeks, coughing fits can become more frequent and severe, even causing vomiting or fracturing ribs.
And they can last up to 12 weeks, which is why it is sometimes referred to as the "100-day cough".
Some older children and adults may not have the characteristic whooping noise, caused when gasping for air.
This is why microbiologist Laurence Luu, from the University of Technology Sydney, says people should try to test early.
The disease is highly contagious and is easily spread by adults who don't realise they have it.
"It's important to get tested early because there are antibiotics that can treat it but they need to be taken early," he said.
"If you're worried, wear a mask and isolate until you get tested."
The disease is most dangerous for babies, with about one in three under the age of 12 months being hospitalised, Dr Luu said.
Newborns may not develop the cough but may experience pauses in breathing and turn blue.
Meanwhile, some babies may have difficulty feeding and can choke during a coughing spell.
People over 65 are also at higher risk of severe cases of the disease.
"The immune system is so busy attacking whooping cough that it also makes you more vulnerable to other secondary respiratory infections," Dr Luu said.
It can lead to pneumonia, brain damage and sometimes death.
About one in 125 babies under the age of six months with whooping cough dies from pneumonia or brain damage, according to the Australian Institute of Health and Welfare.
Why we're having an outbreak
Large outbreaks of whooping cough are cyclical, occurring about every three to four years, Dr Luu said.
After a large infection cycle, more of the population will have immunity, but that will start to wane after a few years.
"You'll see sort of a spike in cases, and it'll go down for a few years and then come back up," he said.
The last major spike was in about 2017, so the next was expected in about 2020 but was delayed due to the COVID-19 pandemic.
And now we're seeing a larger surge in cases.
"It's pretty much on track to be the worst year for whooping cough since 2015–2016," Dr Luu said.
Australia's largest reported outbreak since the widespread introduction of whooping cough vaccines was the epidemic between 2008 and 2012.
More than 140,000 cases were recorded during this period, with the number peaking at almost 40,000 in 2011.
Peter Richmond, head of paediatrics at the University of Western Australia, said it appears Australia is on its way to the next epidemic.
"It certainly looks as though that's the way we're going," he said.
"It's fairly typical of pertussis to have these epidemic periods."
According to the health department's National Notifiable Disease Surveillance System (NNDSS), Australia has already recorded about 11,473 whooping cough cases this year.
Currently, Queensland and New South Wales have so far seen the largest spike in cases.
"We're just now starting to see the numbers are also going up in Victoria, and starting to increase in the other states as well," Professor Richmond said.
"This is really something that started in the second half of last year, and is now really causing problems, particularly in school-aged children."
In previous outbreaks, younger children were mostly getting infected.
This time, cases have been particularly high in children aged 10–14, and now more children under age one are being infected.
Dr Luu said missed routine whooping cough vaccinations at the height of the pandemic, and lack of exposure to the disease, may mean certain groups are more vulnerable.
It's recommended children are given six doses of a whooping cough vaccine between the ages of about six weeks and 13 years old.
"It's a real wake-up call that we need to make sure that we get our vaccinations on time," Professor Richmond said.
The threat of new strains
Australia isn't the only country experiencing a spike in cases.
Whooping cough has been making a post-pandemic comeback in the US, UK, Europe, and China.
China reached more than 30,000 cases in the first two months of 2024, nearly the total number recorded for the entire previous year.
And it recently discovered whooping cough strains which are now resistant to antibiotics.
Dr Luu said right now it's too early to tell which strain is causing the outbreak in Australia.
Research he conducted with UNSW in 2020 found a new strain emerged in the country during the 2008 epidemic, which may have contributed to the large number of cases.
The "smarter" strain could only be targeted by two of the vaccine's components, which meant the disease had a better chance of survival regardless of whether a person was vaccinated or not.
"In Australia, we use a three-component vaccine, which targets three proteins of the bacteria," Dr Luu said.
"There is a five-component vaccine around, but that's not widely used in Australia ... hopefully with those extra components we can target these strains better."
Professor Richmond said there are several developments being tested for new pertussis vaccines that will have longer-term immunity and stop transmission.
He has been working with the Telethon Kids Institute on a nasal spray to prevent the bacteria from causing an infection in the first place.
"The current vaccines are really about stopping the disease, rather than eliminating transmission," he said.
"There's a number of vaccines coming down the line that may appear in the next few years, and be better than the current vaccines."
Time to get a booster?
Dr Luu and Professor Richmond stressed that the current vaccines are still effective, and the best way to stay protected.
As of 2021, more than 95 per cent of five-year-old children in Australia were fully vaccinated, including against whooping cough.
And coverage for the adolescent booster dose was at about 87 per cent.
With the outbreak expected to worsen, people are being advised to ensure they are up-to-date with vaccinations and boosters.
"Pertussis is not going to go away, we know that, and I think this current resurgence is an example of that," Professor Richmond said.
Pregnant women in particular are advised to get a vaccination so some of that immunity can be passed onto their newborns.
It is also recommended that people over 65 and anyone who comes into regular contact with babies get another jab.
"Adults, in general, if it's been more than sort of 10 years since your last vaccine, it's a good idea to get vaccinated to protect yourself, but also protect those around you," he said.
Boosters can also be given at five-year intervals without harm.
Whooping cough vaccines are provided free to pregnant women, and children up to the age of four as part of the National Immunisation Program.
Vaccines are also offered free to adolescents aged 12 to 13 through school immunisation programs.
