I'll try to be as thorough as I can (for an internet post) to explain why you need to take age into account.
Let's start here, with information that we all probably know by now:
Risk for COVID-19 Infection, Hospitalization, and Death By Age Group
If the 18-29 years old age group is taken as the reference group for comparison, every decade you add to that increases the risk of hospitalization and mortality. The risk of hospitalization is 2x greater and the risk of death is 4x greater in 30-39 year olds, in 40-49 year olds it's 2x and 10x, in 50-64 year olds it's 4x and 30x, in 65-74 year olds it's 5x and 90x, in 75-84 year olds it's 9x and 220x and in over-85s it's 15x and 570x.
Meanwhile, given the increased risk to older people, older people are also more likely to get vaccinated than younger people.
According to CDC data
, 81% of those over 75 are fully vaccinated. 86% of those from 65-74 (the most highly vaccinated age range). Below that, the vaccination rate drops with each age range: 74% for 50-64, 66% for 40-49, 57% for 25-39, 53% for 18-24, 52% for 16-17, and 45% for 12-15.
So the median age for vaccinated people will be quite a bit higher than that of unvaccinated people.
Here's another CDC report:
SARS-CoV-2 Infections and Hospitalizations Among Persons Aged ≥16 Years, by Vaccination Status — Los Angeles County, California, May 1–July 25, 2021
Among hospitalized persons and persons admitted to an intensive care unit, the median age was higher among vaccinated persons (median = 64 years, interquartile range [IQR] = 53.0–76.0 years; median = 64 years, IQR = 54.0–76.0 years, respectively) and partially vaccinated persons (median = 59, IQR = 46.0–72.0; median = 65, IQR = 57.0–80.0, respectively) than among unvaccinated persons (median = 49, IQR = 35.0–62.0; median = 56, IQR = 41.0–66.0, respectively) (p<0.001). A lower percentage of fully vaccinated (1.2%) and partially vaccinated (2.0%) persons were admitted to a hospital after their SARS-CoV-2 positive test result date compared with unvaccinated persons (4.2%). A lower percentage of deaths (0.2%, 24) occurred among fully vaccinated persons than among partially vaccinated (0.5%, seven) and unvaccinated (0.6%, 176) persons (p<0.001). Death investigations determined that six of the 24 fully vaccinated persons who died had immunocompromising conditions, including HIV infection, cancer (i.e., prostate, pancreatic, lung, or leukemia), and liver transplantation, and that the median age was higher among vaccinated (median = 78 years, IQR = 63.5–87.5 years) and partially vaccinated (median = 74, IQR = 58.0–80.0) persons than among unvaccinated persons (median = 63, IQR = 51.5–79.5) (p = 0.01).
I hope the above is understandable. The median age of vaccinated people who were hospitalized was 64, compared to 49 for unvaccinated people who were hospitalized (15 year age difference). The median age of vaccinated people who died was 78, compared to 63 for unvaccinated people who died (also a 15 year age difference). And some were immunocompromised to begin with.
It's almost like being vaccinated reduces one's risk to be comparable to that of someone 15 years younger.
Here's the per capita age-adjusted rates:
Among all Los Angeles County residents, the age-adjusted 7-day incidence and hospitalization rates increased exponentially among unvaccinated, fully vaccinated, and partially vaccinated persons, with the highest rates among unvaccinated persons in late June (Figure 1). On May 1, in unvaccinated persons, the age-adjusted incidence (35.2 per 100,000 population) was 8.4 times and the age-adjusted hospitalization rate (4.6 per 100,000 population) was 10.0 times the rates in fully vaccinated persons (4.2 and 0.46, respectively). Partially vaccinated persons had a similar incidence (4.1) and hospitalization rate (0.27) as fully vaccinated persons. On July 25, the age-adjusted incidence in unvaccinated persons (315.1) was 4.9 times that in fully vaccinated persons (63.8); the rate among partially vaccinated persons was 46.8. The age-adjusted hospitalization rate in unvaccinated persons (29.4) was 29.2 times the rate in fully vaccinated persons (1.0); the hospitalization rate was similar in partially vaccinated persons (0.90) (Supplementary Table; https://stacks.cdc.gov/view/cdc/109087).
So in summary, to understand how effective the vaccines are, you need to adjust for age and per capita, because vaccines are not evenly distributed among the population.
I hope what I'm saying makes sense. :notsure:
On the other hand, it does appear that vaccines do not always prevent infection (and transmission) per se, but they do reduce the number of hospitalizations and deaths.
COVID vaccines cut the risk of transmitting Delta — but not for long
The first study to look directly at how well vaccines prevent the spread of the Delta variant of SARS-CoV-2 brings good news and bad.
The study shows that people who become infected with the Delta variant are less likely to pass the virus to their close contacts if they have already had a COVID-19 vaccine than if they haven’t1. But that protective effect is relatively small, and dwindles alarmingly at three months after the receipt of the second shot.