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Experts say more effective masks are vital in protecting each other from Omicron. (Image: Tina Tiller)
Experts say more effective masks are vital in protecting each other from Omicron. (Image: Tina Tiller)

ScienceMarch 9, 2022

No, catching omicron is not inevitable and avoiding the virus still makes sense

Experts say more effective masks are vital in protecting each other from Omicron. (Image: Tina Tiller)
Experts say more effective masks are vital in protecting each other from Omicron. (Image: Tina Tiller)

As omicron continues to surge it’s easy to feel defeatist about your chances of staying ahead of Covid. But ‘let it rip’ still isn’t the answer, as two experts on viral outbreaks explain.

Aotearoa New Zealand has entered new Covid territory, characterised by high vaccination rates but also the rapid spread of the omicron variant and rising numbers of hospitalisations.

As we approach the peak of this wave, some have suggested it would be better to drop remaining public health measures, let the infection rip through our population and accept nearly all of us will get infected very soon. This is unwise for many reasons.

First, simple measures we can all take will ensure that even in this big wave of infections, most of us can still avoid getting infected. Even if you share a household with an infected person, international studies show the risk of catching the virus is somewhere between 15% and 50%.

Second, not all infections are equal.

The delta variant is still circulating and we can’t presume all infections are omicron. While less virulent than delta, omicron can nevertheless cause severe disease and death, particularly among the unvaccinated who make up 3% of the vaccine-eligible population but 19.4% of hospitalisations.

There are still many vulnerable people in the community we can protect by limiting the spread of the virus and ensuring they are less likely to encounter it.

Another reason to limit potentially infectious contact is that infection is more likely if an individual is exposed to a higher initial dose of the virus. An infection avoided or delayed is always a win as we move closer to even more effective vaccines and improved medical treatments for Covid.

Flatten the curve graphic - 2022 version

Why outbreaks come in waves

The reason we get large wave-like outbreaks that rise and fall quickly is because the virus becomes less able to find people to infect as the outbreak progresses. Crucially, this happens before everyone is infected.

This is related to the R number epidemiologists talk about. R0 is the average number of people an infectious person infects at the start of an outbreak. When R is greater than one, the number of cases increases, when it is below one, it decreases.

As the outbreak proceeds, more and more people get infected and recover. They cannot immediately be reinfected. For example, if R is 2 at the start of an outbreak, meaning each case on average transmits to two others, by the time half of the population has been infected and has recovered, the virus will only transmit to one other.

That is because it “tries” to infect two people but finds that, on average, one has already recovered and cannot be reinfected. In this example, the R number is now effectively 1 and infections will start to fall.

Omicron’s rapid spread

Despite New Zealand’s high vaccination rates, omicron is spreading quickly here, as it has in other countries. There are many elements to this.

Omicron is good at avoiding immunity generated by vaccination and previous infection. We have very high rates of first and second doses, but fewer than 60% have received boosters, and we have a very short history of exposure to natural infection.

These characteristics make us prone to a rapid and large outbreak of omicron. Further, vaccinations, including boosters, are very good at preventing illness, hospitalisation and death, but they don’t prevent infection and transmission quite as well.

This means that even in a highly vaccinated population, you can still get high levels of transmission and infection, but the rates of illness and severe complications will be much lower.

Relaxation of public health measures and the impact of superspreader events may also be contributing to the current picture. Importantly, while the number of infections has increased dramatically with omicron, the proportion of these that result in severe complications is much lower than during the earlier delta outbreak.

Our behaviour helps determine the size of the wave

The earlier cases start to fall, the smaller the overall outbreak will be. If R is 2 at the start of an outbreak, a basic model says around 80% of the population will be infected. If the initial R number can be reduced to 1.5, only 58% of the population get infected.

A graph showing the percentage of the population infected over the course of a closed outbreak for different values of R0.
The percentage of the population infected over the course of a closed outbreak for different values of R0. Calculated using the method described by Ottar N. Bjørnstad in Epidemics: Models and Data Using R, Author provided

Luckily we exert some control over the R number. Measures like mask wearing, good use of ventilation, self-isolation when symptomatic or after a positive test, vaccination, and avoiding crowded indoor areas all work to reduce R and the total number of people who will get infected. Local modeling suggests that depending on how well we adopt these measures, somewhere between 25% and 60% of the population are likely to be infected in this outbreak.

Even when sharing the same household as a case, it is not inevitable everyone else will get infected. Studies from the UK, Denmark and South Korea have all looked at the probability of susceptible people in the same household as a positive case getting infected.

They found with omicron, this probability is somewhere between 15% and 50%. In other words, you still have a better than even chance of avoiding infection through your infectious housemate.

All the measures that work generally to reduce spread also work within a household. Mask up inside, get air flowing through, where possible move the infected household member into their own bedroom and bathroom, and practice good basic hygiene.

The relationship between the initial exposure dose, infection and disease severity is a property of many infectious diseases, including respiratory diseases in humans and other animals.

A recent review concluded that while there is good evidence of a direct relationship between the Sars-CoV-2 virus dose and infection in humans, evidence for a link between dose and severity is lacking, despite some evidence from animal models.

Covid severity is most likely driven by factors other than the initial exposure dose. These include the virus variant and host factors such as age or the presence of some pre-existing health conditions.

All the standard public and personal health measures will help us avoid getting infected and reduce transmission to the more vulnerable, thereby reducing the number of people with severe illnesses.

David Welch is a senior lecturer in computer science at the University of Auckland and Nigel French is a professor of food safety and veterinary public health at Massey University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.

Keep going!
RATs can pick up a Covid infection when viral load is high. Illustration: Toby Morris
RATs can pick up a Covid infection when viral load is high. Illustration: Toby Morris

ScienceMarch 9, 2022

Siouxsie Wiles & Toby Morris: Feeling Covid-positive but your RAT is negative? Here’s why

RATs can pick up a Covid infection when viral load is high. Illustration: Toby Morris
RATs can pick up a Covid infection when viral load is high. Illustration: Toby Morris

A quick guide to why some people who return a negative rapid antigen test result may still be infected – and infectious.

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I’m currently getting a lot of messages from people with Covid-like symptoms who are testing RAT-negative. Obviously, one reason could be that they don’t have Covid, rather some other lurgy instead. With our borders opening and new arrivals no longer having to isolate, we’ll be starting to get viruses like influenza coming into the country once again. 

But there are several reasons why people with Covid-19 can return a negative RAT.  

Before we get into that, a quick reminder: for most of the past two years, New Zealand has relied on PCR testing to identify people with Covid-19. PCR is a very sensitive technique that tests for the genetic material of the virus, which means it identifies people very early in infection when there are only a few virus particles around. While there are some rapid PCR testing devices, these usually only take a small number of samples at a time, so we’ve opted for sending samples to labs around the country to be processed. That does mean that depending on where you live, it can take anywhere from a few hours to a few days to get your result back.

We’ve also focused on using nasal swabs as the sample of choice. That’s despite the efforts of New Zealander abroad Dr Anne Wyllie in showing that saliva is a reliable alternative to nasal swabs. While using saliva does have some disadvantages, for many people it beats having a swab stuck up your nose, so means more people are likely to get tested. 

As omicron started to spread in New Zealand, our PCR testing capacity soon maxed out and we’ve now switched to using rapid antigen tests – RATs – to identify people with Covid-19. The ones we’re using have been validated for swabs taken from a few centimetres inside the nose. Unlike PCR, which looks for the genetic material of the virus, RATs detect specific bits of the virus itself. These are the antigens. This means RATs aren’t as sensitive as PCR, so will pick up infectious people a few days after a PCR test would have done. That’s why it doesn’t make sense to use RATs when case numbers are low and PCR capacity high – people may end up with a false sense of security, thinking they don’t have Covid when they do. 

But when case numbers are high, RATs make perfect sense. They are really quick to do – hence the “rapid”. The tests we’re using give an answer in 15-20 minutes. It’s really important that you don’t read the test after this time as the result will be unreliable.

RATs can also be done pretty much anywhere. That’s one of the disadvantages though: because most tests now aren’t being processed by our testing labs, we are reliant on everyone logging their test result with the Ministry of Health to give us an indication of our case numbers. We also need people to log their negative results, as that will help us keep an eye on the positivity rate, which can show us if we are testing widely enough to catch most cases. To log your result, go to the My Covid Record website or call 0800 222 478.

Below are some of the reasons why people with Covid-19 can return a negative RAT.

The sample wasn’t taken properly

People might not be swabbing the right part of their nose. I’m not judging, have you seen what our noses look like from the inside?! Dr Eric Levi posted a great picture on Twitter that shows where the swab needs to go, which Toby Morris has adapted below. Levi is an otolaryngologist, a doctor whose specialty is the ears, nose and throat, so he knows what he’s talking about. Basically, your nose goes back, not up. So, push that swab low and slow to get it right. 

There wasn’t enough virus in the sample

Let’s say that you’ve followed the instructions and are now an expert nose swabber. Why might you still test negative? As I explained earlier, RATs aren’t as sensitive as PCR tests so you can test negative if there isn’t enough virus around to detect. Normally we would expect people to have plenty of virus present once symptoms start. But for those of us who are vaccinated, especially if we are boosted, our symptoms will start earlier after we’ve been exposed because our immune system is able to recognise the virus much more quickly. It’ll also be working hard to get rid of the virus.

If you have symptoms, and you’ve tested negative, take another test a day later. Hopefully by this time there will be enough virus to detect.

Your nose isn’t the bit of you that’s infected

OK, so let’s say you’ve done several swabs and they are still negative. Congratulations! You may be one of the people who will only test positive through a throat swab or using a saliva sample. A recent study from researchers in South Africa compared paired nasal swabs and saliva samples by PCR and found that about one in 10 nasal samples tested negative for omicron when the corresponding saliva sample tested positive. It was different for delta, where about three in 10 of the saliva samples tested negative when the corresponding nasal sample tested positive.

(Yes, some people have tried swabbing their throat and then their nose with a nasal RAT, but as the swab won’t be designed for the throat, it can be hard to do.)

In conclusion, and I know this is an absolute pain, but just because you’ve tested negative doesn’t always mean you don’t have Covid-19. This is especially true if you have any of the hallmark symptoms like a scratchy throat, headache, fatigue, fever, or a change in/loss of your sense of taste and/or smell. 

If this is you, and you are able to isolate, please do, as this will help reduce the spread of the virus, protect our family, friends and workmates, and keep our healthcare system functioning.

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