The vanishing point

What’s so bad about the Delta variant?

As I write this, Australia is once again facing long lockdowns. The Covid-19 outbreak that gave Wellington a scare a few weeks back has escalated, with 1700 active cases and 5 deaths in Australia so far. Right now, I’m feeling truly grateful that my brother and his family made it back to New Zealand from Sydney when the bubble was open, and I’m feeling sorry for some work colleagues in Melbourne whose travel plans are now on hold. When I think back to the couple of weeks when Wellington went into alert level 2, I realise just how lucky we have been, both here in Wellington, and in the whole of New Zealand.

Australia’s latest outbreak is caused by the Delta variant, which emerged at the peak of India’s second wave, although it was actually first reported late last year. I’ve talked about Covid-19 variants before, when they were still named after the country they were first reported in. If you want to understand the mutation process that forms the variants, and why we keep getting new ones even though Covid-19 has a much lower mutation rate than a virus like influenza, then take a look at that article. But I thought it was time for me to pay more attention to the Delta variant, and to learn why it is proving so dangerous.

The Delta variant is formally known as B.1.617.2, and it’s one of many hundreds of variants now reported on the Pango database, which is a database that tracks variants. While there are hundreds of variants, there are only a few designated by the World Health Organisation as “variants of concern”, that is, variants that have traits making them more transmissible, more virulent or less susceptible to control and treatment. In late May, the World Health Organisation announced a new naming convention for these variants of concern – using letters of the Greek alphabet. That means that the UK variant became the Alpha variant, the South African variant became Beta and the Indian variant became Delta.

The Delta variant has moved fast, and has now been reported from 101 countries. It’s difficult to know how many cases are caused by Delta, though. Britain has reported ten times the case numbers of any other country, a highly implausible situation. Britain routinely tests for variants whereas  many other countries don’t, so the reported high prevalence there is almost certainly because the British are making the greatest effort to look for it. What we do know is that when tests are run to determine what variants are present, Delta is now dominant almost everywhere.

What is it about Delta that’s allowed it to spread so rapidly and become so dominant? A recent study from China, which closely monitored people who were under quarantine after exposure to the Delta variant, provided some answers. The people in quarantine were tested daily and, on average, it took four days for the virus to be detectable in those who later went on to develop the infection. But back at the start of the pandemic, it took six days for the virus to be detectable. At first glance, this appears to indicate that the Delta variant has a disadvantage, not an advantage. After all, the more rapidly the virus can be detected, the more rapidly infected people can be isolated. That idea works in theory, but not in practice. You need to know that someone is potentially infected in order to test them. And tracking down, then testing, the contacts of a known case can take time. So, the more rapidly the virus is building up in an infected person, the less time there is to track down the people who came into contact with that person.

But it isn’t just the speed at which infection develops which is a problem. The same study also found that the amount of virus present in an infected person is much higher with the Delta variant compared to the original virus. This is important because, as a general rule, the more virus that is present in someone, the more likely they are to infect someone else. Australia has seen this, reporting that 100% of people living in the same house as someone infected with the Delta variant become infected themselves. With the original Covid-19 virus, the household transmission rate was only 25%.

These two factors add up to a virus which is more likely to be transmitted sooner after infection. But is there anything else that’s different about the Delta variant? Does it cause more severe symptoms, or evade the immune system more easily? And, perhaps the most important question, will our vaccines still work?

At this stage, the answers to these questions aren’t all that clear. In the UK, there are suggestions that the symptoms associated with the Delta variant may be subtly different. With earlier strains, a fever and coughing were the most commonly reported symptoms, with headache and sore throat reported by some, and a runny nose rarely. Now, the most common symptoms are headache and sore throat, and a runny nose is also being commonly reported. There are also suggestions that the Delta variant affects younger people more, and is twice as likely to put people in hospital than the Alpha variant, although it’s too soon to be sure.

But there is one piece of good news coming out of the UK right now. The study I mentioned in the previous paragraph also found that the Pfizer/ BioNTech vaccine gave good protection against the Delta variant, even though the protection wasn’t quite as good as against the Alpha variant. And that protection wasn’t just preventing people from developing severe disease, it was also preventing people from becoming infected in the first place. This is really good news, because if the vaccine prevents people from becoming infected, they won’t be able to pass the virus on either. And if vaccinated people aren’t passing on the virus, then we have a chance of reaching herd immunity, where there are too few susceptible people in the population to support an outbreak. Herd immunity would protect not just the vaccinated, but everyone.

However, when it comes to herd immunity, the news isn’t good.

The most important number for working out whether we can achieve herd immunity is the virus’s reproduction rate. I’ve talked about herd immunity and the reproduction rate before, so if you want an overview, check out that article. But for a quick recap – the higher the reproduction rate (often shortened to R0) for a virus, the higher the proportion of the population that needs to be vaccinated to achieve herd immunity. The other important factor is the vaccine’s effectiveness at preventing infection, and therefore transmission. Using the reproduction rate of earlier variants of Covid-19, around 3, and assuming a vaccine that was 95% effective in preventing transmission, a vaccination rate of around 70% was required to achieve herd immunity. That’s a realistic figure even if we don’t vaccinate children and have a few people who don’t want to be vaccinated.

But the new variants of Covid-19 are transmitted more easily, which means their reproduction rates are higher. For the Alpha variant, which was dominant in the UK before Delta arrived, the reproduction rate is around 4-5. Delta is worse, and its reproduction rate is somewhere in the range of 5-8. And vaccines aren’t quite as effective at preventing transmission. Based on real-world data and not clinical trials, the Pfizer/ BioNTech vaccine was 92% effective at preventing transmission of Alpha, but only 79% effective at preventing transmission of Delta.

If you put those numbers into the equation for calculating the vaccination rate needed for herd immunity, there’s some bad news. Even if we use the lower reproduction rate of 5, the rate of vaccination needs to be 100% to achieve herd immunity – which means, in effect, that the promise of herd immunity has vanished. The situation is going to be worse in other countries like Australia and the UK, because the Oxford/ AstraZeneca vaccine is significantly less effective at preventing transmission.

The news isn’t all bad. Cases of severe disease are rare in people who have had both shots of the Oxford/ AstraZeneca vaccine, and even rarer in those who’d had the Pfizer/ BioNTech vaccine. We must vaccinate as many people as possible, as soon as possible.

As I write this, I have one nagging doubt. Officially, I will be able to book for my vaccination from mid-late August, according to the current timetable. Among other developed nations, this is late – places like the UK and Italy have around half of their populations fully vaccinated now, and New Zealand has just 13%, as you can see in the graph below. It’s tempting to complain, until you put those figures into context with countries that have a much greater need than us – Indonesia, South Africa, Nigeria and Papua New Guinea, for example. Right now, Indonesia has Southeast Asia’s worst Covid-19 outbreak. South Africa continues to suffer. Nigeria is struggling, with many states simply not testing for Covid-19 at all. And Delta has just arrived in Papua New Guinea, where vaccination rates are shockingly low.

When I am notified to book for my Covid-19 vaccination, I will definitely be doing so. We all need to do our bit to protect New Zealand as well as we can. Whatever the variant, the more of us who are vaccinated, the harder it will be for Covid-19 to spread in the community. But, when I get my shots, I will know that I am very lucky indeed.

Postscript: you can help support vaccination in lower income countries by donating, for example to the Go Give One campaign, which is backed by the World Health Organisation, or to UNICEF.


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