Competition between syringes.
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Current vaccines were designed to work against the SARS-CoV-2 coronavirus that originated in Wuhan, China.
But as the virus evolves around the world, new variants are emerging that seem to evade the human immune system. Several strains are concerning for public health because they’re more transmissible and/or more deadly — notably the Alpha, Beta, Gamma and Delta variants first detected in the UK, South Africa, Brazil and India.
People are being given a variety of different vaccines to help trigger immunity, slow the spread of Covid-19 and stop outbreaks. Are existing drugs effective against variants?
While there are many ways to measure whether a vaccine is effective, here I’ve focused on a figure most often highlighted in scientific studies: the proportion of people that a drug has protected from developing moderate to severe disease.
For preventing Covid, that percentage is usually calculated after accounting for both symptomatic and asymptomatic infections, which produces a real-world figure for a vaccine’s effectiveness.
This article covers four available vaccines: ‘ChAdOx1 nCoV-19’/AZD1222 developed by University of Oxford/AstraZeneca and Ad26.COV2.S by Johnson & Johnson/Janssen — which use viral vectors to deliver Coronavirus spike proteins to prompt immune responses — and BNT162b2 by Pfizer/BioNTech and mRNA-1273 by Moderna, two drugs based on mRNA technology that deliver the genes for making spikes.
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I’ve collected data from a dozen sources: research published in The Lancet and New England Journal of Medicine (NEJM), plus an as-yet unpublished report from Public Health England (PHE) on how well the AstraZeneca and Pfizer vaccines work against the Delta (B.1.617.2) variant.
Effectiveness of vaccines against SARS-CoV-2 variants (two doses).
Data: Lancet, NEJM, PHE / Chart: JV Chamary (CC BY 4.0)
The bar chart above shows each vaccine’s effectiveness or ‘efficacy’ at preventing Covid caused by variants (no data doesn’t mean zero efficacy).
All figures are approximate and should be treated as such. That caveat aside, all four vaccines are highly effective against the original virus but less so against each ‘Variant of Concern‘ (I didn’t find reliable data on Gamma/P.1).
Although data on Moderna’s vaccine were excluded because they don’t appear in peer-reviewed research, it uses similar technology to Pfizer’s drug and so results are expected to be comparable.
Almost every data point shows at least 50% efficacy, which is reassuring because it’s the minimum set by two influential regulators for the use of pharmaceutical products, the European Medicines Agency and Food & Drug Administration.
The one exception that falls below the 50% threshold is AstraZeneca’s efficacy against the Beta variant (B.1.351). That data point comes from a study in South Africa, which found that the drug was only 10.4% effective and so “did not show protection against mild-to-moderate Covid-19.”
Although the study didn’t assess severe disease, that effectiveness was low enough for Oxford to launch a trial of a new vaccine — AZD2816 — that’s been redesigned for protection against Beta.
Testing whether a vaccine work is challenging, especially against variants. One major challenge is that, in a given country, several strains can be circulating in the population at the same time.
Early studies didn’t test people for variants, so you have to assume that a vaccine’s effectiveness reflects how well it worked against the original virus — Wuhan-Hu-1, a viral isolate with the notable D614G mutation — rather than new strains of SARS-CoV-2.
The Alpha (B.1.1.7) variant was first identified in late 2020, for example, so the 70% figure for AstraZeneca (from an interim analysis in December, when Alpha was not yet in widespread circulation) is probably for efficacy against the Wuhan-Hu-1 strain.
Recent research has tested for variants. Johnson & Johnson’s vaccine was 74% effective in the US, where over 96% of Covid cases were caused by the original SARS-CoV-2 (suggesting efficacy against Wuhan-Hu-1), and 52% effective in South Africa, where 95% of cases were due to Beta.
By contrast, an unpublished study of Mayo Clinic patients from early 2021 is less reliable as sampling suggests Alpha was prevalent in between half and two-thirds of Americans over that period.
The chart above shows figures from people who are considered ‘fully vaccinated’. For vaccines that require two doses for a complete regimen, that means one jab to prime the immune response and a second booster shot to reinforce immunity later. Johnson & Johnson’s drug is a single-dose vaccine (so bars don’t appear in the chart below).
How effective are two-dose vaccines when people only receive the first shot and are ‘partially vaccinated’?
Comparing the figures for ‘two doses’ (both/single) with the chart below — for effectiveness against the original virus after ‘one dose’ — illustrates the importance of completing a regimen: efficacy is lower for almost every drug.
Effectiveness of vaccines against SARS-CoV-2 variants (one dose).
Data: Lancet, NEJM, PHE / Chart: JV Chamary (CC BY 4.0)
For effectiveness against a variant after one dose, few data were available and most come from studying Pfizer’s vaccine, which is slightly (albeit not significantly) more effective against Alpha than the original virus and less effective against the Beta and Delta variants.
Interestingly, two different technologies — AstraZeneca’s vector and Pfizer’s mRNA-based vaccine — offer equal protection against the Delta variant after the first drug dose. That 33% efficacy means, on average, two out of three people who are partially vaccinated will be susceptible to infection by Delta and could develop Covid.
The implication of such a low figure is worrying, as those statistics suggests that it’s premature for governments to be ‘opening up’ countries — to relax controls such as mask wearing, social distancing and travel restrictions — if their vaccination program hasn’t yet reached the majority of a population.
Even people who have been fully vaccinated against the original strain may not have sufficient immunity against variants — you can’t be “immune to Coronavirus” — and will catching then transmitting Covid to those who are more vulnerable to the potentially deadly disease.
To end on a positive note, however, it seems that most existing drugs offer close to 100% protection from hospitalization and death.
In one way then, existing vaccines are effective enough against variants.
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