[set fair (OP)]

Boris (and presumeably SAGE) says the fall in cases is largely due to lockdown.
Tim Spectre (and presimeably Blofeld) says it's vaccination.
What does our motley crew have to say?

[charles1948]

Boris (and presumeably SAGE) says the fall in cases is largely due to lockdown.
Tim Spectre (and presimeably Blofeld) says it's vaccination.
What does our motley crew have to say?

The fall in cases is probably due to warmer weather, as we go from Spring into Summer.

But when we go from Summer into Autumn, then Winter, and get colder, the cases will increase again.

[Kryptid]

Surely it's some combination of the two.

[charles1948]

Surely it's some combination of the two.

Yes, possibly the vaccines may have some minor effect.  But  the major effect is probably caused by seasonal temperature variations.

[Bored chemist]

The fall in cases is probably due to warmer weather, as we go from Spring into Summer.

Last year the number of cases rose as we went form mid march to mid April. This year, it has fallen.
So we know it isn't the weather.

[Petrochemicals]

The shutting of schools. Cases took ages to comedown, yet the majority of cases where in people who where active and not vaccinated, by default of being in work.

[set fair (OP)]

I think the worrying possibility is that it is the same unknown factor which caused the abrupt fall in the R number in the US leading to the peak on January 9th and rapid fall in US cases.

I think that the shutdown is more resposible for the fall than vaccination. We only reached 30% (first jabs) on march 1st - enough to counter the reported increased transmisability of the UK variant. Add 2 or 3 weeks for the most recently vaccinated to achieve some immunity and vaccination as the major reason doesn't seem plausible.

[evan_au]

Last year the number of cases rose as we went form mid march to mid April.

Commenting from a distance: This time last year, the virus was in exponential growth phase, and pandemic precautions like mask wearing, social distancing and contact tracing mechanisms were not yet in place.
- Now these pandemic processes are in place, and provided they remain actively in operation, spending less time locked up inside together is probably good for people.

[alancalverd]

The virus lives in humans, where the temperature is a constant 35 - 37 deg C, so it isn't affected by the weather.

It is transmitted between humans, so the rate of secondary infection depends on the density of human interaction.

It takes 3 - 5 days to replicate to a significantly transmissible level and is only reported (due to persistent symptoms) some 5 - 10 days after infection, so the number of cases reported on any day depends on the density of human interaction one to two weeks earlier, by which time the number of infections ha increased according to the subsequent intensity of human interactions.

Hence the expected peaks after Chinese New Year, Thanksgiving (for what, one might now ask?) and Borisday (a compulsory national shopping day a few weeks before Christmas).  Cheltenham Races was almost certainly responsible for a minor outbreak in Ireland but the number of sources was too small for the UK response to be discernible above the ridiculously high background.

Intentional infection, by discharging infectious patients from hospitals to nursing homes, caused a uniquely British peak and should be prosecuted as a war crime.

Remember the variation in R number has nothing to do with the virus and everything to do with the behavior of the vector - people. And always allow for the peculiarities of the reporting "system": most people recover from fairly mild symptoms, so won't present to a hospital or a doctor on a fine, sunny day when you can  take a sickie and sit in the garden. But if you feel like crap and it's raining, why not take a bus to the hospital and infect a few more people?

[Bored chemist]

The virus lives in humans, where the temperature is a constant 35 - 37 deg C, so it isn't affected by the weather.

The "lifetime" of the virus in air and on surfaces is strongly influenced by UV intensity, humidity and temperature.
Because of this, viral transmission is affected by weather.
Virus that is in the 37^oC oven ism't being transmitted. Transmission requires that it leaves the body.

Intentional infection, by discharging infectious patients from hospitals to nursing homes, caused a uniquely British peak and should be prosecuted as a war crime.

Since there was no war, this should be prosecuted as an ordinary crime. Thousands of cases of murder or manslaughter- we can let the lawyers decide which (as long as they do it quickly).

[Petrochemicals]

The virus lives in humans, where the temperature is a constant 35 - 37 deg C, so it isn't affected by the weather.

I'd have to say the humidity is probably the factor Alan, Manaus in the humid rainforest was very well hit. The virus is not airborne, just by water droplets in the air, condensation is a signifier of air saturation. We had a poor summer and it failed to destroy the virus, but we did have a dry sunny spring in 2020.

[alancalverd]

Transmission requires that it leaves the body.

and enters another before it is inactivated by drying or UV. So it's all dependent on population density. I guess exhaled aerosol may remain active a little longer in cold weather but it mostly evaporates in a couple of seconds.

Much of the goings-on in Syria and formerly in the Balkans  has been classified as war crimes even without a formal declaration of war against the civilian population.  I might settle for corporate manslaughter if those responsible will admit that their action was incompetent rather than  a deliberate affront to scientific data and common sense. However as HM Govt was simultaneously advocating social distancing, the action of infecting nursing homes must have been intentional, discriminatory, and thus presumed malicious. 

[Bored chemist]

and enters another before it is inactivated by drying or UV. So it's all dependent on population density.

Well... no.
Because it is partly about humidity and UV (among other things), it is not "all about"  anything, is it?

[CliffordK]

I had hoped that COVID would be knocked down last summer, and it may have had lower transmission than it would have had if it was cold.  Our fall/winter peak was certainly much higher than the summer peak.

If one looks at past pandemics, like the 2009 Swine Flu, the disease continued to spread throughout the first summer, but mostly disappeared by the second summer (with good vaccinations too).  However, it circulated enough that every few years it would make it back around.

One of the big thing that affects COVID spread is Human activities, such as schools, colleges, parties, etc.  And our activity patters may well be seasonal.

I'm hoping that people will start choosing lower risk activities throughout the summer, although that would also depend on the location.  So, around here, it is nice outside most of the summer (although we may have a hot summer this year), but in other states the hot weather drives everyone inside to the air conditioning.

Oregon has had somewhat lower infection rates than the rest of the USA, but we are heading into a strong fourth wave of infections,with greater pressure to reopen the schools.  So far the mortality rate has been low, but that is a lagging indicator, so time well tell if we can keep the mortality low.

We are up to about 1/4 the Oregon population vaccinated, with another 1/8 in progress.  But, that means we still have a long way to go.