Mu vs. Delta. Does it call for a Delta booster?

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Mu vs. Delta: An Update

Last month I covered Mu— a new SARS-CoV-2 variant the World Health Organization announced as a “Variant of Interest”.

Briefly, Mu was first discovered in Colombia in January 2021. It since spread across the globe, including the United States. There was considerable interest in this variant because, as the WHO stated, it has a “constellation of mutations that indicate potential properties of immune escape”. In other words, there are number of changes on the virus in which our treatments and vaccines may not recognize, and, thus not work.

On August 31, the WHO announced it was closely watching how Mu competed with Delta in Colombia and Ecuador. Can Mu outcompete (i.e. more transmissible) Delta? Because if it can, we may be in trouble.

What has unfolded since?

In Colombia, Mu became the dominant variant in May 2021. In June, Delta was introduced and, in recent weeks, has been slowly pushing Mu out of the way.

In Ecuador, Mu arrived in May 2021 and has been holding steady for the past three months. But another data source (second graph below) is showing a much different picture in Ecuador. Recently Mu started increasing and pushing Delta away. According to this data, 50% of cases in Ecuador are Mu and 50% of cases are Delta.

There is a significant challenge with following variant dominance in Ecuador: They have a very small genomic surveillance program. From August to September, only 12 samples were sequences (compared to the UK which sequences 10,000 per week). So we have to take uncertainly into account. When we do that, the “true” prevalence of Mu ranges from 0-60% of cases. This is a really big range, so we’ll just have to continue to see how Mu vs. Delta plays out in Ecuador.

What about the United States?

In North America, Mu was introduced in April 2021 and started spreading quickly. However, Delta came along in June 2021 and quickly became the dominant variant by August. In the United States, specifically, the CDC is now reporting zero cases of Mu. Delta accounts for 99.8% of cases.

We’re seeing this in the science too.

One team found Mu (red) was, on average, more fit than previous variants. Compared to Delta (green and teal), Mu is not as fit. But this is on average; there is significant overlap in our certainty (some red dots overlap with the green/teal dots).

So, Mu isn’t an immediate threat for the United States. (Thankfully?) Delta was introduced. If it wasn’t, Mu would certainly be the dominant variant right now and, possibly, escaping our vaccine and natural protection.

Going forward, we need to focus on how Delta is changing:

  • Is it becoming more transmissible? There is a point at which transmissibility caps out.

  • Is Delta mutating to eventually escape our vaccines? And, if so, how quickly? This doesn’t cap out. Viruses can change slowly over time (like Measles that doesn’t mutate at all) or fast over time (like the flu).

As Dr. Trevor Bedford (a brilliant scientist that assesses viral dynamics) said:

“So far, there is little signal [of Delta changing] but I should expect such sub-lineages to emerge in the coming months”.

Since Delta is the threat, should we wait for a Delta-specific booster?

Pfizer is testing a vaccine that specifically targets Delta. Results from the clinical trials are anticipated in the forth quarter of 2021. Moderna also announced that it’s testing a Delta vaccine (and, more interestingly, a combo vaccine that targets both Beta and Delta).

So, should we wait for a Delta booster? No. For four reasons:

  1. We don’t know if we need them at all. There are some studies that show a third dose of the same vaccine formula is just as good as a third dose of a vaccine with an adjusted formula.

  2. If we will need them, we certainly don’t know when. While we expect the variant to mutate, we don’t know if it will mutate enough to warrant another vaccine.

  3. If we needed them today (which we don’t), it would still take weeks (if not months) to get the data, go through regulatory approval, and distribute the new vaccines across the country.

  4. We do know that vaccine efficacy is suboptimal for immunocompromised. We also know that efficacy is waning. Right now. Especially for those 65+ or got the vaccine more than 6 months ago. So, if you’re in a high risk category, get the original booster formula. We need you protected now.

Love, YLE

P.S. Thanks to Dr. Bedford for the Mu reminder yesterday on Twitter! And, as always, his brilliant analyses and contribution to the field.