Waning immunity combined with a changing virus means that even if you’re fully vaccinated and/or have recently had omicron, you shouldn’t be throwing caution to the wind this winter.

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Back when the mRNA Covid vaccines were first rolling out, I explained the two important reasons why “fully vaccinated” might mean needing more than two doses. The first is that we didn’t know how long immunity from two doses would last. In other words, our immunity from vaccination could drop over time – something experts call waning immunity. 

These were a new type of vaccine – based on the virus’s genetic material rather than the whole virus or just its proteins – and targeted against a virus we’d never encountered before, so there was lots of uncertainty. But it wouldn’t be a surprise to experts as lots of the other vaccines we use routinely need more than two doses, and for some, the doses are spread out over years to counter waning immunity. For example, here in Aotearoa New Zealand, we get offered a vaccine that protects us against diphtheria, tetanus and whooping cough when we’re six weeks old, three months old, five months old, and then again when we’re four, 11 (or 12), 45 and 65. Anyone pregnant will also be offered a dose in their second or third trimester.  

The second reason we thought two doses might not be enough was the potential of the virus to change, meaning the immunity we did have would no longer be as strong against the new variants. Again this is something we already know happens with other infectious microbes. The best example is probably the influenza virus, which changes so fast and so much that we need a new flu vaccine every year. 

The Covid-19 double whammy

Unfortunately, with Covid-19 it turns out we’re dealing with both waning immunity and the virus evolving at the same time. It’s a double whammy. 

The data is really clear that immunity after two vaccine doses wanes within a few months of the second dose, which is why a third “booster” dose was offered and why it really isn’t just an optional extra. It now looks like immunity from that third dose wanes too, though it’s still not fully clear by how much another dose might bring that immunity back up again and how long it will last. However, the data is also clear that some people definitely need more than three doses, which is why fourth doses have been offered to some groups and fifth doses will soon be made available.

Alongside waning immunity, the virus has also been changing. In November last year, the BA.1, BA.2 and BA.3 versions of omicron appeared. Omicron was more infectious than delta and had the ability to infect people who had either been vaccinated or had been infected with earlier versions of the virus. That means omicron soon took over, causing Covid cases to surge all around the world. 

And with more transmission, the virus has had lots of opportunities to evolve even further. Now we have new, even more infectious versions of omicron, called BA.4, BA.5 and BA.2.12.1 – all evolved from BA.2. These are now causing new surges in Covid cases overseas.

If you’ve had omicron, you still need to be cautious

Overseas data is showing that some people who had omicron a few months ago are starting to get reinfected. That could mean the immunity they got from having Covid is waning, just like it does after vaccination. Or it could be due to changes in the new omicron variants. My bet would be that it’s a combination of these two things. 

What’s important to understand, though, is that most other places in the world had a BA.1 wave followed by a much smaller BA.2 wave. In Aotearoa, we had the opposite. Both BA.1 and BA.2 seeded here at similar times, but BA.2 quickly became dominant. That means the majority of people who’ve had omicron here have had BA.2. As I said earlier, the new omicron variants are based on BA.2 so what we don’t yet know is if people who’ve had BA.2 are less likely to get reinfected with these new variants than people who’ve had BA.1. At least until the virus changes again.

We’ll find out soon enough though, as BA.4, BA.5 and BA.2.12.1 have all been detected in the wastewater in Aotearoa, which means they’re circulating in the community. If they’re able to outcompete BA.2 then we’ll see BA.2 start to disappear over the coming weeks. And if they can easily reinfect people who’ve had BA.2, then we’ll start to see cases rise again.

My advice is that even if you’ve had Covid recently, you shouldn’t think you’re invincible and stop taking precautions. It’s really clear this isn’t a “once and done” virus.     

The winter wave

Experts are predicting that we’re likely to see a surge in Covid cases over winter, thanks to our waning immunity and the arrival of the new omicron variants. With our borders reopening, we’re also starting to see an increase in flu. Some of our hospitals are already struggling, limiting visitors and postponing elective procedures. That means the months ahead could be really tough. We’re also expecting the microbes that cause measles and whooping cough to arrive too, which have the potential to cause devastating outbreaks as a lot of our little kids aren’t up to date with their routine vaccinations.

Here’s how we can all help. If you haven’t had your Covid booster dose yet, get it. Talk to your health provider and see if you qualify for a free flu jab. If you don’t qualify and can afford it, then I recommend you get it. If you are an employer, think seriously about funding the flu jab for your staff. It’ll mean fewer people off sick over winter and less disruption to your business. 

If you aren’t up to date with your kids’ routine vaccinations, then talk to your health provider about catching up. This is really important. Measles and whooping cough are so dangerous, especially to babies who are too young to be vaccinated.

Keep up the good habits you’ve learned that reduce the transmission of Covid. Wear a good-quality mask when indoors. Open doors and windows, even just a little, to improve ventilation. Get tested if you have any symptoms, and stay home and isolate if you have symptoms/test positive.