US Covid-19 Immunity Is Among the Worst Globally

Introduction

Booster programmes are underway in most countries we track (Table 1). Israel’s programme started in July, and the country has already triple-dosed nearly 45% of its population. Next is Chile at 35%. But no other country on our list has passed 16%, and globally, just under 2% of the population have received their booster. So how important are these booster programmes?

Using the latest studies on vaccine efficacy, we show how quickly immunity is changing across the world. The actual level of immunity, which we call effective immunity, gives a real-time estimate that balances declining two-shot immunity with immunity-restoring booster programmes. And data from Israel shows just how important these booster shots will be entering winter.

Effective Immunity

Research by the ZOE COVID Study and results published in Nature Medicine provide estimates and simulations of waning vaccine efficacy. For the Pfizer-BioNTech vaccine, efficacy falls to 74% after six months and below 60% after nine months. This rate of decline will significantly impact a country’s effective immunity.

Israel first administered vaccines in December 2020. By July, the total protection these initial doses provided had dropped 10pp (Chart 1). Without a booster programme, this figure would have doubled by now, and effective immunity would be at late February/early March levels. Instead, the boosters built on the protection of the two-dose strategy, pushing effective immunity beyond its April 2021 peak.

Israel’s Summer Covid-19 Wave

Israel is one of the few countries to release daily vaccination data by age group. This allows us to run the same estimations of effective immunity on the most vulnerable groups. By July, effective immunity had fallen 13pp from its peak, reducing aggregate protection among the over 50s by almost 20% (Chart 2). Simultaneously, weekly deaths started to increase among the same age group (Chart 3).

Crucially, though, the link between cases and deaths remained weaker than in previous peaks. In January’s wave, an average of three people per 100 cases died in the over 50s age group (versus 0.8 for the whole population). In the recent wave, this figure declined to 1.6 in the over 50s and 0.4 for all age groups (Chart 4).

However, the ratio of deaths to cases increased notably in the fifth week of the July wave. Around three in every 100 people aged over 50 who caught Covid-19 in mid-July (one week after the booster programme started) died, only 0.5 fewer than at the equivalent stage in the January wave. This ratio then declined much more quickly than in the January wave, once the booster programme kicked in.

Eliciting the marginal impact of a decline in immunity on deaths is difficult. But the above is evidence that the link between deaths and cases strengthened as immunity declined. In this simple analysis, the booster effect in Israel is worth around 0.7 deaths per 100 cases in the over 50s, based on a counterfactual where the deaths-per-cases ratio follows the same dynamic as in January from the sixth week onwards.

Under this assumption, the booster programme has saved 360 lives in the 50+ age group, reducing the total number of deaths by 30%. We can apply the same calculations to the total population, where the booster programme is worth 0.2 deaths per 100 cases. This has avoided an estimated 870 deaths up to the end of October or 37% of total deaths.

Elsewhere

How are the US, UK and EU faring? All three have begun booster programmes. In the US, around 8% of the population have received a booster, increasing the immunity trajectory only slightly (Chart 5). Overall immunity there is among the bottom 25% of countries on our list. Meanwhile, the UK is further through their booster programme, which has pushed effective immunity up to 57% (Chart 6). Lastly, with the EU having started later, two-dose immunity is yet to decline significantly (Chart 7).

Imperial College London recently estimated where deaths in the UK could be this winter. According to their researchers, deaths could be four to six times higher without boosters. While this is far higher than the 30-37% of lives saved in Israel, this includes additional winter pressures and assumes immunity will have fallen much lower, especially in the most vulnerable groups. Nevertheless, it reaffirms the importance of booster programmes entering winter.