Letter: When we don't know

Submitted by AWL on 16 February, 2021 - 7:14 Author: Martin Thomas

As the World Health Organisation puts it, “No substantive data are available related to impact of [the AstraZeneca vaccine] on transmission or viral shedding”. Or other vaccines.

There are theoretical reasons to hope that the vaccines reduce transmission a fair bit. Getting to know definitely will be difficult. Solid studies of transmission require good knowledge about where each new infected person got the virus.

That’s why we still don’t know how much less never-symptomatic people transmit than eventually-symptomatic ones, and how small a factor transmission via surfaces is, rather than through particles in the air (probably much smaller than first thought, but…)

Meanwhile there is agitation for “vaccine passports” and the like. It lacks scientific grounding.

As Zack Muddle (letters, Solidarity 581) says, the scientists’ committee which decided the vaccination schedule in Britain included health and care workers alongside the old and vulnerable in the priority groups. Health and care workers face much more risk than other occupational groups.

The committee cited a secondary argument, as Zack quotes: “Even a small reduction in transmission arising from vaccination would add to the benefits” here, because health and care workers are also at more risk of transmitting to the old and vulnerable.

Note: even, small, would, add to… The scientists recognise they don’t know about reduction in transmission.

Probably by the time two-dose vaccination of groups prioritised for their risk of Covid death or severe illness is complete (a way off yet) we will know more about transmission, and that can be factored into later prioritisation. We don’t know yet.

Martin Thomas, Islington


Submitted by Zac Muddle on Mon, 22/02/2021 - 17:20

Martin Thomas's response (Solidarity 582) implicitly although indirectly acknowledges my slight corrective (Solidarity 581) to his reporting of the vaccination schedule. His response to my central claim mostly sidesteps it. His original article (Solidarity 580) implied — if I read it rightly — a level of difficulty in assessing whether the vaccines can reduce transmission “a lot” which is overstated and unfounded. While acknowledging that would it would be difficult, I attempted to redress the balance.

There was no implication that we do know already. The central thrust of Martin's response, that we don't know yet is irrelevant.

Perhaps I should have been, or could if I was writing it now, have been stronger.

In science, people often distinguish between “accuracy” and “precision”. Accuracy is how close a measurement or estimate is to the “true” value. Precision is how narrow an estimate's range can be, or how repeatable an experiment may be. I may claim that the speed of light is 100,000-900,000 km/s. Or I could claim that it is 299,999 km/s. The latter claim is more precise: it gives the speed of light to the nearest km/s, rather than within a very wide range. But the former claim is more accurate, as it encompasses the actual (accurate and precise) value: 299,792 km/s (to the nearest km/s).

It will be, for the reasons I stated, very difficult to get any precise figures on how much any of the vaccines reduce transmission. Getting broad-brush understanding, which isn't precise but can be relied upon — i.e. is accurate — will be considerably easier.

What constitutes reducing transmission “a lot”? An over 50% reduction (within a given timeframe) certainly would. Depending on what you want to know for, I would have thought even a 20% reduction would constitute “a lot”.

Research has found that “the viral load of index cases was a leading driver of SARS-CoV-2 transmission”  Preliminary data shows that some vaccines can significantly decrease viral load. Add to my previously cited study, more recent (not yet peer reviewed) ones, for example this, or on viral load this.

The respective trials claiming a reduction of over 80% in infections, in the second “89.4 percent… whether symptomatic or not”. The third finds a four-fold decrease in viral loads of those infected, due to the vaccine. Soon, if not now, it will be reasonable to make accurate although not precise claims that some vaccines do reduce transmission “a lot”, e.g. by between 30% and 100%.

As of the 10 February (the day after my initial letter), the CDC updated their advice. “Fully vaccinated persons who meet criteria [no symptoms, within 3 months of the vaccine] will no longer be required to quarantine following an exposure to someone with COVID-19”. While this measure seems premature to me, their argument for confidence of a significant reduction seems persuasive. “Although the risk of SARS-CoV-2 transmission from vaccinated persons to others is still uncertain, vaccination has been demonstrated to prevent symptomatic COVID-19; symptomatic and pre-symptomatic transmission is thought to have a greater role in transmission than purely asymptomatic transmission.”

Zack Muddle, Bristol

Submitted by Martin Thomas (not verified) on Mon, 22/02/2021 - 17:55

My points in Solidarity 580 were that:

1. "vaccines are not the cure-all". They do not abolish the need for covid-distancing, quarantines, or even (for a good while yet) lockdowns.

2. "the carefully-reasoned elderly-first vaccination schedule" designed by the scientists should not be rejected in favour of a priority for young and fit people on the grounds that the vaccines probably cut transmission, and those young and fit people socialise more than the elderly and vulnerable.

I conceded that "the vaccines probably reduce transmission to some degree, maybe a lot". I argued only that we should not rush to "vaccine passports" and the like, and that the vaccines' more proven value in reducing deaths should not be sidelined in favour of a speculative program of vaccinating the young and fit first in the hope that will kill transmission.

The figures from Indonesia, where the government did decide to vaccinate the younger and fitter first, seem to bear that out.

I'll be as glad as anyone if it turns out that (all?) the vaccines suppress transmission (of all variants?) heavily. But, as the CDC report quoted by Zack says: "there is currently limited information on how much the vaccines might reduce transmission..."

Submitted by Zac Muddle on Mon, 22/02/2021 - 23:11

I very explicitly did not, and do not, disagree with the points as made by Martin in the comment above. My previous points all still stand.

Add new comment

This website uses cookies, you can find out more and set your preferences here.
By continuing to use this website, you agree to our Privacy Policy and Terms & Conditions.