Does Vaccinations Plus Cases Equals Population Immunity in Arizona?

 

On Monday I did a post worrying about Arizona's falling vaccination rate.  I showed a curve suggesting that if things keep going the way they are now, the percent fully vaccinated might level-off well before we get to 50% of the population.

In a conversation about that with some public health people I know, it was pointed out that we also have to take previous cases into account when considering the percent who make have some level of immunity.  Having had and recovered from COVID-19 give you some immunity, but not as much as if you're fully vaccinated.  The thinking seems to be that having a case is somewhat similar to one dose of a two-vaccine regimen.

But there are problems with rolling infection-based immunity together with vaccinations to estimate what percent of the population is protected.  For one thing, many people who have had COVID-19 have also gotten vaccinated.  Some researchers I know who recently completed a nationwide study told me that about 35% of people who had or thought they COVID-19 reported also having vaccinated.

Another problem is that not every case of COVID-19 is reported.  Some researchers I know who have been looking at serology studies (looking for evidence of infection in blood samples taken for other reasons) estimate there are 4.5 unreported infections for every reported infection.

So based on this, we can do some back-of-the-envelope calculations of the percent of Arizonans who have some level of immunity. The ADHS website says that 812,742 cases have been confirmed in the state.  Based on the serology numbers, there are probably an additional 3,657,338 unreported cases, for a total of 4,470,081 suspected/reported cases.  

As I reported on Sunday, 45.65% of the Arizona population, or 5,718,183 people, have received a dose. But according to the survey results described above, 35% of those represent people who have already been infected.  Subtracting those from the vax doses leaves 3,716,818 people who have not been infected but have received a dose.  

That means 8,186,899 Arizonans have some level of protection.  What's the problem with that number?  The state population is only 7,424,805. So much for back-of-the-envelope calculations!  

But the point is, a lot more people in the state have some level of protection than the pure vaccination numbers would indicate. One public health person I know estimates the number at 60%-70%.  

So maybe I was over-worried in Monday's post. This would explain why our infection rates are so low, and continuing to fall, even though we're only around 35% fully vaccinated.

Header image by Gerd Altmann from Pixabay 

Popular posts from this blog

Looks Like Immune Responses are Enduring After All

Image
 For some time, public health officials have fretted about whether immunity to COVID-19, acquired via infection or vaccination would stick.  Studies showed rapid declines in antibodies during recovery from infection and thereafter . It turns out that this concern was overblown. A study just released says bone marrow plasma cells are a persistent source of protective antibodies, and that "circulating resting memory B cells directed against the S protein were detected in the convalescent individuals."   Another study , yet to be reviewed, suggests that vaccines boost this effect. In fact, some are suggesting this situation this could actually mean  lifetime immunity .   Whether the same thing happens with vaccination is not clear. Most estimates suggest that vaccine immunity will last about a year . More pessimistic estimates putting the protection period at six months .  The possible shorter time frame, plus the appearance of the new variants, have the vaccine makers working
Read more

Another One Bites the Dust

Image
Hi all. Stepping out of retirement for a moment to say that the day has finally arrived. After four years of successfully avoiding COVID-19 infection I had a positive test yesterday. Symptoms are very mild—I've had worse colds. The provider at urgent care said almost all the cases they are seeing are the same. Mild symptoms may also be due to the fact that I had the original vaccine then the two boosters. All this time I thought I might be a NOVID —one of those people who never get the bug. But no, it must have been my assiduous mask wearing and avoidance of high-risk situations when things were bad. So here's double middle fingers to all the kooks who think masks don't work (I've not been wearing masks for months now). I don't know what variant I have but it's likely JN.1. It is said to be extraordinarily contagious and accounts for 60% of cases worldwide . It also tends to produce mild infections like the one I have.  This article claims that right now every
Read more

AZ Pandemic Numbers Summary for the Seven Days Ending November 9: Everything is Going South

Image
Here are the Arizona pandemic numbers and graph of statewide cases for the seven day period ending yesterday: Well this isn't good. The lull seems to be over. Virtually every number is up, both over a week and over four weeks. The most worrisome is hospital beds, which have been steadily trending up over the last several weeks.  Wastewater signals are all up except Tempe (dashboard is up again) which after going up last week (from 41 to 100) decreased by about 10%. The increases are worrisome because the latest data from ASU's Lim Lab (last updated October 31), where they sequence PCR samples to identify variants, shows that only about 46% of sequences showed BA.5. That is one of the Omicron variants the new bivalent vaccine was designed to protect against.  All the rest of the samples are BA.2.75 (7.7%), which I flagged several weeks ago, and BQ,1.1 (15.4%), and BQ.1 (30.8%), both of which I blogged about a couple weeks ago.  Those, new research confirms , are much more lik
Read more