Explained: How coronavirus compares to flu

Experts have warned that New Zealand's health system could face a "double whammy" if Covid-19 happens to spread during our already busy influenza season. But what's the difference between the two infections? And which poses the bigger problem? Science reporter Jamie Morton explains.

What are the basic differences between them?

Influenza and the fast-spreading coronavirus dubbed Covid-19 are both respiratory tract infections caused by an RNA virus – or one which has ribonucleic acid as its genetic material.

But the coronavirus driving Covid-19 is more closely related to the virus that fuelled the global severe acute respiratory syndrome (Sars) outbreak during 2002 and 2003.

Although it isn't yet clear whether Covid-19 is influenced by environmental factors – or spread faster over the colder months – research into previous human coronaviruses have indicated they may peak in winter.

How do the symptoms vary?

They're similar in that both can cause fever, cough, body aches, fatigue, and sometimes vomiting and diarrhoea, said Dr Lisa Maragakis, a senior director of infection prevention at Johns Hopkins University in the US.

Both could also be mild or severe – even fatal in rare cases – and result in pneumonia.

The Ministry of Health points out that, because the symptoms are similar, feeling them doesn't mean you have Covid-19.

"Both infections can be relatively mild, or make you feel terrible," said Professor Mick Roberts, an infectious disease modeller at Massey University.

"I have had the flu and spent a week in bed feeling like a truck has run over me, but it isn't always that bad.

"It appears that you can have Covid-19 without symptoms, but then the case fatality ratio for Covid-19 is much higher than that for influenza."

Of more than 100,000 Covid-19 confirmed cases so far, there have been around 3500 deaths. More than 57,000 people have recovered.

Around one billion cases of the flu are reported worldwide each year, causing between 291,000 and 646,000 deaths.

In New Zealand, the flu is estimated to kill about 500 Kiwis each year – especially men, Māori, Pacific Islanders and those living in poverty – and infects 10 to 20 per cent of the population.

What spreads faster?

The flu has a basic reproduction number – or the average number of people who catch the virus from an infected person - of 1.3.

Early studies have suggested that value for Covid-19 could be between two and three.

Both can be spread from person to person through droplets in the air from an infected person coughing, sneezing or talking.

Flu can be spread by an infected person for several days before their symptoms appear, and Covid-19 is believed to be spread in the same manner.

A possible difference is that Covid-19 might also be spread through the airborne route.

Other early indications from China are that Covid-19 may be behaving similarly to Sars, which ultimately infected 8,098 people and caused 774 deaths across 17 countries – a fatality rate of 9.6 per cent.

But that picture could change, as there is still much for scientists to learn about it.

"We need to estimate the serial interval, the average time from you being infected to you passing it on; and the basic reproduction number, the average number of people you infect," Roberts said.

He noted Sars had a longer serial interval and a higher basic reproduction number than influenza.

"Observations of the serial interval early in an epidemic are not based on a random sample, and are often heavily biased.

The Ministry of Health is urging people to cover coughs and sneezes with disposable tissues. Photo / 123RF

"You can only estimate the basic reproduction number from the growth rate in cases, but this depends on knowing the serial interval. A shorter serial interval and a lower basic reproduction number will give you the same growth rate.

"So, Covid-19 could be behaving more like influenza as far as transmissibility goes, you can't rule that out."

How are each treated?

Neither virus can be treated with antibiotics, which only work on bacterial infections.

Both may be treated by addressing symptoms, such as reducing fever, and severe cases may require hospitalisation and support such as mechanical ventilation.

Various antiviral medications, which have been found to sometimes shorten the duration of flu, were currently being tested to see if they can address symptoms of Covid-19.

New Zealand annually offers a flu vaccine – this year's covers four strains, including H1N1 and H3N2-like viruses, but one for Covid-19 is still in progress, and may be up to 18 months away.

And how are they prevented?

Both may be prevented with measures like frequent, thorough hand washing and staying home when sick and limiting contact with people who are infected.

The ministry urged people to cover coughs and sneezes with disposable tissues, and to wash hands for at least 20 seconds with water and soap and drying them thoroughly before eating or handling food, and after using the toilet, coughing, sneezing, blowing your nose or caring for sick people.

"Hand washing is very important," Roberts stressed.

"The viruses can be transmitted through small droplets in the air or on surfaces, and can enter the body through the eyes, nose or mouth. Your hands can transfer the virus to your face."

People who suspect they have Covid-19 should call a dedicated healthline for free on 0800 358 5453 – and phone ahead before visiting a GP clinic or hospital.

What happens if both hit at once?

"You would be unlucky to get both at the same time," Roberts said.

"The major implication would be the stress on the health system, with two epidemics with similar symptoms and the need for lab testing, isolation and care."

The ministry's director of public health, Dr Caroline McElnay, acknowledged a flu season outbreak "would be a challenge" and needed to be considered in planning.

But she expected New Zealand's Covid-19 experience would be "pop-up cases" able be quickly quarantined to avoid further spread, as had been so far managed here and in Australia.

That said, public health experts have questioned whether New Zealand's system could withstand a double-hit – one noting that the country recently ranked only 35th for pandemic preparedness.

The Government has activated its national pandemic plan and poured $3m into a "rapid research" fund to address the threat and better prepare New Zealand.