Vaccinate for the holidays

The Covid vaccine is safe and effective and it’s good that most eligible people are getting it. But how much protection does it give? If you look at the NZ statistics on who gets Covid, it seems to be extraordinarily effective: the chance of ending up with (diagnosed) Covid for an unvaccinated person is about 20 times higher than for a vaccinated person.

That’s probably an overestimate. People who are vaccinated are at lower risk for immunological reasons: the vaccine really works.  We’re also at lower risk for social reasons: if you’re vaccinated, your friends and family and people you interact with are also more likely to be vaccinated, so they are less likely to give you the virus. That’s partly due to equity problems in the vaccine rollout and partly just to what social-network people call homophily:  you tend to hang out with people similar to you. The immunological reason will hold true over summer; the social reason perhaps less so if people travel. 

Also, because elimination came so close to working in Auckland, the virus has been fairly effectively suppressed in most of the New Zealand population.  On top of the clustering of unvaccinated people, there’s very strong clustering of the current outbreak — it’s mostly in Auckland, but it’s not at all evenly spread within Auckland.  Even if you’re in Auckland you probably know either no-one or lots of people who have been infected.  If you’re in know no-one, you’re at lower risk– and you’re probably vaccinated. As we go from a more or less localised outbreak to many little outbreaks, this additional clustering will go away and the apparent benefit of vaccination will fall.

How much will it fall (and why am I sure)? In the USA, you’re currently about 6 times as likely to get a Covid diagnosis if you’re unvaccinated (according to the CDC). In the UK, the ratio comparing unvaccinated people to those with a Pfizer vaccination within four three months is 4-5.  That fits with the estimates of how effective the vaccine is, biologically, against Delta, plus a bit of social clustering.  The ratio in NZ will be heading that way over time.

So: vaccines, yes, but also masks and distancing and meeting people outside when you can and getting tested if you have symptoms and not going to isolated places that don’t even have enough of their own health care.  Don’t give the virus an inch.