Friday, October 08, 2021

Covid, Vaccination and Infection

Judging by current evidence, vaccines continue to provide strong protection, about 90%, against death or hospitalization, for many months. Protection against infection is much weaker. A recent study in Qatar found that the Pfizer vaccine's protection against infection got down to about 20% by the fifth month. That is the lowest figure I have seen; other estimates of the effectiveness of vaccination against infection over time vary, but 50% is fairly typical.

If the question is whether I should get vaccinated in order to protect myself, the main issue is effectiveness against hospitalization or death. If the issue is whether you should get vaccinated to protect me and others, on the other hand, the main issue is by how much vaccination reduces the chance you will get infected and be contagious.

People are likely to take fewer precautions against getting infected after they are vaccinated — certainly I did. If effectiveness is down to 20%, even 50%, the chance of infection might easily be as high among the vaccinated as the unvaccinated. Infection of the vaccinated is more likely to be asymptomatic, in which case the individual is unlikely to know he is infected, hence unlikely to take precautions against transmitting the infection, which increases the chance of transmission. It is, so far as I can tell, unclear how close the relation is between severity of infection and contagiousness — at least I have seen claims both that asymptomatic people are much less contagious and that they are not.

I conclude that while it is clear that by getting vaccinated I protect myself against consequences of Covid, it is not clear, contrary to what the CDC, Biden, and a lot of other people claim, whether and how much I protect other people against the risk of my catching Covid and passing it on to them. It is that claim on which the current push to require people to be vaccinated is based. 


Some evidence that while vaccination may reduce contagion it doesn't reduce it very much is the lack of any clear connection in international data between vaccination rate and infection rate. Israel had steeply increasing infection rates from late June to early September, the UK from late May to late July, periods in which both had relatively high vaccination rates — higher than Czechia, which had declining rates until late June, roughly constant for the next two months, and much higher than India, which has had a declining infection rate from early May to the present. I have not yet seen any careful analysis of the international data trying to separate the effects of climate, vaccination rate, and natural immunity from previous infection, but the effect of vaccination alone is not large enough to be obvious from a casual look at the data.


At 2:15 PM, October 08, 2021, Blogger jg said...

'Infection of the unvaccinated is more likely to be asymptomatic' citation? I see no reason to expect any difference; your theory is I guess that the vaccines reduce severity of symptoms when infected.

At 11:00 AM, October 09, 2021, Blogger Average Radical said...

Preliminary evidence out of Israel questions herd immunity even further:

"The attack rate among exposed individuals reached 23.3% in patients and 10.3% in staff, with 96.2% vaccination rate among exposed individuals. Moreover, several transmissions probably occurred between two individuals both wearing surgical masks, and in one instance using full PPE, including N-95 mask, face shield, gown and gloves.
It challenges the assumption that high universal vaccination rates will lead to herd immunity and prevent COVID-19 outbreaks. This was probably true for the wild-type SARS-CoV-2 virus, but in the outbreak described here, 96.2% of the exposed population was vaccinated. Infection advanced rapidly (many cases became symptomatic within 2 days of exposure), and viral load was high. Another accepted view is that, when facing a possible mismatch between the SARS-CoV-2 variant and vaccine or waning immunity, the combination of vaccine and face mask should provide the necessary protection. Although some transmission between staff members could have occurred without masks, all transmissions between patients and staff occurred between masked and vaccinated individuals, as experienced in an outbreak from Finland [12]."

At 6:02 PM, October 09, 2021, Anonymous Anonymous said...

Erratum:'Infection of the unvaccinated is more likely to be asymptomatic' - probably intended to be: 'Infection of the VACcinated is more likely to be asymptomatic'. - Which could lead indeed to higher rates of infection, as an asymptomatic carrier would be less likely to test and more likely to go out and be be less extra-careful than a symptomatic one, obviously. On the other hand, one probably has no symptoms because of a much lower viral load, and in vaccinated a positive test might have reacted to the terminated left-overs of the virus. In the end, it matters little: Get the shot, protect yourself. If all do that, all have pretty good protection. See the super-low death rates in Israel and the UK despite high infection rates. Corona is down to/below traffic-deaths. Trafic in the 90s not in the very bloody 70s.

At 11:49 PM, October 11, 2021, Blogger David Friedman said...

You are correct that it should have been "infection of the vaccinated." I have now fixed it.

At 10:40 AM, October 12, 2021, Blogger The Blog 101 Team said...

This comment has been removed by the author.

At 2:43 PM, October 12, 2021, Anonymous Mark P said...

Professor Friedman - You may be interested in the ongoing data from the UK, summarized in one chart here: The original data for the latest week: (Table 2). Tables 3 and 4 have emergency care and death rates, which reflect the improved outcomes of the vaccinated, as expected. This lends support to your intuition that vaccine value is captured by the recipient and not the group.


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