About 19% of recent documented COVID-19 cases in California are breakthroughs, and state data shows that those who have been fully vaccinated account for an increasing portion of positive tests.
The number, which contradicts a repeated public portrayal that breakthrough cases are negligible, can be easily misinterpreted. To be clear, this is not an indication of some sort of vaccine failure. Quite the contrary.
Breakthrough cases were expected. State data still suggests that unvaccinated people are nearly five times as likely to be infected as those who are inoculated. And almost all the hospitalizations and deaths are among unvaccinated people. Vaccines remain the most important tool for fighting the pandemic.
Rather, the rising proportion of breakthrough cases suggests that even people who have been vaccinated are potentially significant spreaders of coronavirus, especially the delta variant. It reinforces why vaccinated people should also wear masks in public settings.
Breakthrough case rates are a sensitive topic, one that some health officials are trying to avoid and which has sparked a lot of handwringing in the media about how to report it.
The fear is that misinterpretation of the numbers will dissuade people from getting vaccinated. But, without the data, the important push for everyone to wear masks is weakened.
Although vaccinated people are well-protected against serious illness, they can, if infected and even if they’re not showing symptoms, spread the virus to others. Indeed, people with breakthrough infections of the delta variant might be just as contagious as unvaccinated people, although the science continues evolving on that point.
The call for vaccinated people to don masks is not just for their own protection. “It’s not about you, it’s about everybody,” says Contra Costa County Health Officer Chris Farnitano. “It’s about keeping the cases down.”
But, to make that point, it’s critical to have the supporting data.
Unfortunately, the U.S. Centers for Disease Control and Prevention stopped tracking breakthrough cases on May 1, focusing only on patients who are hospitalized or die. The thinking was apparently that the risk of transmission from breakthrough cases was negligible. We now know, with the delta variant, that’s no longer the case.
This past week, much media attention has misleadingly focused on a Kaiser Family Foundation survey of breakthrough data from the 24 states that are tracking breakthrough cases. It found that the cumulative infection rate for vaccinated people since the start of the year is well below 1% in all reporting states. Thus, the narrative has downplayed breakthrough cases.
But cumulative 2021 data is not helpful information for confronting the current surge driven by the relatively new delta variant. Which is why the daily breakthrough case rates are important.
To their credit, California and many county health officials have been tracking new COVID-19 cases and cross-checking them with state vaccination lists to determine the infection rates of vaccinated and unvaccinated.
Thus, for the week of July 31, the statewide average daily rate for vaccinated people was 7 per 100,000, double what it was two weeks earlier, and for unvaccinated it was 33 per 100,000.
But, amazingly, state health officials won’t say what portion of all the new cases involve vaccinated people. It’s not rocket science. We know the portion of the population that has been vaccinated. With the numbers the state has already released, the rest of the calculation is simple math.
Officials from Santa Clara, Alameda and Contra Costa counties, who track the same data for their jurisdictions, have confirmed my calculations or provided their own. The state, for three days this past week, refused to do either.
In Contra Costa County, the seven-day average as of July 28 showed that 23% of the new cases were breakthroughs in fully vaccinated people. For Santa Clara County for about the same time period, the portion was 16%. Alameda County estimates 19% of new cases for the month of July were breakthroughs. And, doing the math, the portion for California was 19% as of July 31, up from 15% about two weeks earlier.
State officials note correctly that the portion of breakthrough cases would be expected to increase because more people are getting vaccinated. But the increase is outpacing the rate of newly vaccinated.
To be sure, the data, like most of the COVID-19 case data we have, is limited because it relies on test results. Thus, it probably underrepresents breakthrough cases because infected vaccinated people are more likely than unvaccinated to be asymptomatic and consequently less likely to seek testing.
Randomized testing is needed to understand the prevalence of breakthrough cases more accurately. But now that we have surpassed 10,000 daily cases in California, it’s no longer insignificant that roughly one in five are from those who have already completed their shots.
It’s time for everyone to wear masks.