The tipping point
Is herd immunity for Covid-19 a realistic goal?
There’s no doubt that countries which learned from previous disease outbreaks are among the most successful in handling Covid-19. Media accounts reporting the success of countries like Singapore, Taiwan and South Korea usually credit lessons learned during outbreaks of diseases such as SARS in 2003 and MERS in 2015. While New Zealand is definitely among the successful countries, I have not seen any suggestion that our success resulted from a previous, and less successful, encounter with an infectious diseases. But then I began to research herd immunity and vaccinations, and came across the example of measles.
Measles is one of the most contagious diseases to infect humans. It’s also deadly. Before a vaccine was developed in the 1960s, it killed more than 2.5 million people a year. Decades after a vaccine was introduced, it was still killing tens of thousands – around 140,000 in 2018, mostly young children. Most deaths are in developing countries, but even in wealthy countries it kills one in every thousand infected. And even when it’s not fatal, it’s a nasty disease. In New Zealand, around 15% of people who get measles require hospital treatment. The hospitalisation rate, combined with its rapid spread, means that measles has something in common with Covid-19 – it can overwhelm a country’s health system if allowed to get out of control.
The vaccine against measles is one part of the MMR (measles-mumps-rubella) vaccine, which, in New Zealand, is given to children at 12 months, with a second dose at 15 months (although until a few months ago it was given at 15 months and 4 years). MMR is a “live” vaccine, which means that it contains weakened forms of the measles, mumps and rubella viruses. The viruses in the vaccine are viable, and capable of reproducing in the human body, but have lost most of their ability to cause human disease because they’ve been grown for many generations in a laboratory. If the idea of live vaccines makes you feel a little uncomfortable, I can’t blame you. Instinctively, I feel as if a live vaccine can’t be as safe as a vaccine that’s been completely inactivated, or made from just a part of a virus. But my instincts aren’t correct. There is a huge amount of research on the safety of the MMR vaccine, and it has far fewer side effects than, for example, the Pfizer/ BioNTech Covid-19 vaccine that I discussed last week. There’s a good summary of the side effects, and a link to even more information, on this page from the US Centres for Disease Control.
As well as being a very safe vaccine, MMR is also very effective, particularly against measles. With two shots, the vaccine is more than 96% effective against measles, and gives lifetime protection against the disease. (MMR isn’t quite as effective against mumps and rubella; for those diseases it’s about 90% effective and doesn’t give lifetime immunity.) But it doesn’t do anything if it just sits in a vial in the fridge – and that’s where there’s a problem.
I’ve talked before about vaccination rates in New Zealand, and the fact that they have been declining in the last few years. At the end of 2018, around 88% of five year olds had received all their recommended vaccines – that’s below the threshold for herd immunity for highly contagious diseases such as measles. But the figures are actually worse than that, because the 12% of unvaccinated children are not spread evenly throughout New Zealand. Some regions had vaccination rates over 90%, while others, such as the Bay of Plenty and the West Coast, were below 80%. More deprived areas also had more unvaccinated children, although, in reality, no area of New Zealand had enough people vaccinated to prevent an outbreak of measles taking hold.
And, in 2019, that’s exactly what happened. By the end of September 2019, New Zealand had gone from having eliminated measles to the second highest rate in the western Pacific, exceeded only by the Philippines. By November, we had exported our outbreak to Australia, Samoa, Tonga and Fiji.
If we aren’t able to get vaccination rates high enough to achieve herd immunity for measles, which is a well-understood disease with a highly effective vaccine, what are our chances with Covid-19?
The concept of herd immunity first came to the attention of the wider public in the early days of the pandemic. Back then, some governments suggested that the best strategy for Covid-19 was simply to let the disease run its course, without imposing the kinds of control measures that would affect the economy. Eventually, they suggested, enough people would become immune and the disease would fade away. But virologists have pointed out that herd immunity isn’t a likely outcome of uncontrolled spread of disease. This can be seen in one of the worst-affected areas of Brazil, the city of Manaus. A study which tested for antibodies to the virus – an indicator of past infection – showed that up to 66% of the population had been infected by October of last year. The study suggested that this figure may be around the level required for herd immunity, and when the outbreak waned in May and stayed low for some months, it looked as if they may have been right. But, in December, case numbers rose. The city’s hospitals and cemeteries were, once again, overwhelmed.
Herd immunity is not a usual outcome of the uncontrolled spread of disease. But it is something that’s achievable with widespread vaccination. The idea behind herd immunity is that there are so few people in the community who are susceptible to a disease that an outbreak cannot take hold. An infected person coming in from outside is unlikely to encounter someone who is susceptible, so the disease doesn’t spread. As a result, everyone in that community is protected, even those who cannot be vaccinated, or those who don’t have a strong enough immune system, like people undergoing cancer treatment or very young babies.
There are various numbers that have been tossed around as the threshold for herd immunity with Covid-19. The article which wrongly suggested herd immunity may have been reached in Manaus gave a figure of 67%. The figure of 70-85% appears in a number of media articles, such as this one from NPR in the US. But the reality is that we just don’t know.
The proportion of immune people needed to achieve herd immunity to a disease depends on the reproduction rate of that disease. The reproduction rate (often shortened to R0) is the average number of people that one infected person passes the virus on to. It’s not an absolute number – it depends on both traits of the virus and how the population as a whole is behaving. If everyone is out and about, frequenting shops, bars and restaurants, not keeping their distance and generally acting as if everything is normal, the virus’s reproduction rate will be higher. If everybody stays home, the rate drops. The higher the reproduction rate, the higher the threshold required for herd immunity.
New Zealand didn’t have herd immunity for measles with vaccination rates averaging around 88%, because measles is extremely contagious. While estimates vary widely, commonly quoted figures suggest that each person infected with measles will infect somewhere from 12-18 other people. In the case of Covid-19, that figure is lower – a review of a number of different publications gave an average reproduction rate of around 3. However there was a great deal of variation in the results depending on circumstances. On the Diamond Princess cruise ship, in an outbreak which sickened 619 of 3700 passengers and crew, the reproduction rate was nearly 15. In Italy, early in their outbreak, the number was around 2.5-3, while in South Korea the number was estimated at 1.5 or even less.
There’s a mathematical equation for working out the proportion of the population that needs to be immune to achieve herd immunity. The equation is relatively simple (for those who like equations, it’s 1-(1/R0), and it is within my mathematical abilities, so I’ve tried out some examples to see what different reproduction rates tell us about herd immunity. If, for example, the reproduction rate is a rather conservative 2.5, then, in theory, only 60% of the population needs to be immune to reach a point where an outbreak is unlikely. If it’s at the average quoted figure for Covid-19, around 3, then 67% of the population needs to be immune. If the rate is much higher, say 10, then 90% need to be immune. And if the reproduction rate is at the level of the Diamond Princess outbreak, around 15, then 93% of people need to be immune.
When it comes to achieving herd immunity with Covid-19 vaccines, though, the percentage of people who are vaccinated is not the percentage who are immune. Not all vaccines are equally good at protecting people, and this has implications for the proportion who need to be vaccinated. The effectiveness of the vaccine is another number which has to be added in to the calculations. If we are talking about the Pfizer/ BioNTech vaccine, which is around 95% effective, then for our reproduction rate of 3, we need 70% vaccinated, a not-unrealistic number. But if we are talking about the Oxford/ AstraZeneca vaccine, with an efficacy somewhere around 70%, we need 95% vaccinated. Suddenly, herd immunity doesn’t seem like such a realistic goal.
And those figures assume the average reproduction rate for Covid-19. Push that number up a bit, perhaps for a city where people live in very crowded conditions, and the numbers don’t look good at all. If we take a reproduction rate of 4, and the AstraZeneca vaccine, we start getting nonsense numbers. The equation tells me that we need to vaccinate 107% of people, which is clearly impossible. With the Pfizer/ BioNTech vaccine, we can still achieve herd immunity, with 79% vaccinated, but it’s starting to get more difficult.
There are other problems as well. We don’t yet know how long immunity will last, and we won’t for some time. If immunity only lasts a couple of years, that doesn’t give much time to eliminate the virus before people are susceptible again. So we’ve got to get people vaccinated quickly, and re-vaccinate.
And there’s another very important piece of information that we just don’t know yet. As I mentioned in last week’s article, the companies developing the vaccines worked out how effective they are by looking at how many vaccinated people got Covid-19, compared to the number who people who got sick in the group who got the placebo. Using that method, it’s not possible to tell whether the vaccine is preventing people from catching the virus, or simply preventing people becoming sick from the virus. That’s a really important distinction when it comes to herd immunity. If the vaccine stops people from becoming sick, but doesn’t prevent them catching and spreading the virus, then herd immunity won’t be possible. Vaccinated people will be protected, but the unvaccinated, and those who are immune-compromised, will still be at risk.
All of this has forced me to the rather depressing conclusion that herd immunity for Covid-19 may not be something that can be achieved easily, if at all. There’s not going to be a tipping point, when Covid-19 just goes away. Instead, it looks as if vaccination is going to be just another tool to help shift the balance, alongside hand-washing, staying home when sick, physical distancing, wearing masks, testing and isolating, and contact tracing. If those measures, all of which help reduce the reproduction rate, are used alongside widespread vaccination, the numbers suggest that Covid-19 could be eliminated without resorting to severe lockdowns. But it’s not going to be easy.
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