Coronavirus vaccine breakthrough cases are rare in Colorado. Here’s everything we know about them.

Fewer than 2% of Colorado’s coronavirus cases since mid-January have occurred in people who are fully immunized against the virus — another sign the vaccines are offering strong protection against COVID-19 in the state.

Nisha Alden, the COVID-19 surveillance manager for the Colorado Department of Public Health and Environment, said Wednesday that 2,916 people in the state have tested positive for COVID-19 after being fully immunized. Since Jan. 15 — the approximate date when people who were first in line to receive the vaccine in Colorado reached full immunization — there have been 168,944 total cases of COVID-19.

That means about 1.7% of the state’s coronavirus cases since Jan. 15 have been in a fully vaccinated person. Looking at it another way, about 0.1% of people in Colorado who are fully immunized have developed COVID-19.

Given what the state knows about the vaccines’ effectiveness and about the virus’ rate of spread in Colorado, state officials say the low numbers are in keeping with expectations.

“It’s a very small proportion of all the vaccinated people and all the cases,” Alden said.

Deaths and hospitalizations are also low among the vaccinated

Fully immunized people have accounted for about 2% of COVID-19 hospitalizations and about 3% of deaths since mid-January, according to CDPHE figures.

Vaccine breakthrough cases have accounted for 218 hospitalizations and 38 deaths, Alden said. There have been more than 10,400 COVID-19 hospitalizations and more than 1,100 COVID-19 deaths since Jan. 15.

Alden said those suffering from severe coronavirus infections after vaccination are usually those most vulnerable to COVID-19 in general — people who are older or who have serious underlying medical conditions.

“COVID is one of their causes of death but not always their primary cause of death or hospitalization,” Alden said.

There isn’t any particular variant causing the breakthrough cases

CDPHE officials have been conducting genetic sequencing on the virus samples from breakthrough cases to determine if there is any variant of the coronavirus that appears especially adept at punching through the vaccines’ protection.

But, so far, nothing has stood out, Alden said.

As of mid-May, close to 70% of Colorado’s new COVID-19 cases were estimated to be caused by variant forms of the coronavirus that federal authorities have designated as “variants of concern” — meaning they are believed to be more dangerous than older forms of the virus. The B.1.1.7 variant — first identified in the United Kingdom and now sometimes called the Alpha variant — accounted for about 60% of all new cases in mid-May.

Those percentages are slightly less than in late-April and early-May, when upwards of 85% of new cases were estimated to be caused by variants of concern. CDPHE says one reason for the decline may be the rise of another variant — B.1.617.2, which was first identified in India and is now also known as the Delta variant. It is not labeled a variant of concern by the U.S. Centers for Disease Control and Prevention, though the WHO has declared it one. Therefore, CDPHE does not include B.1.617.2 in its estimates for cases caused by variants of concern.

As of last week, B.1.617.2 had been found in 11 Colorado counties and was believed to account for more than 20% of new cases in the state.

Health officials stress that, even if the vaccines are somewhat less effective against the coronavirus variants, they still work well. Alden said that’s showing up in CDPHE’s analysis of breakthrough cases.

“Right now the vaccine is effective against all variants,” Alden said.

Where vaccinations are high, hospitalizations are low

Another sign that the vaccines are doing a good job of beating back COVID: The counties that are doing the best on vaccination rates also have the lowest hospitalization rates.

During a virtual news conference Wednesday, Alden showed a chart plotting counties based on the percentage of their eligible population that is fully immunized and on their COVID hospitalization rates. The result? A notable decline in hospitalization rates as vaccination rates tick up.

The Colorado Sun conducted a similar analysis, instead using two-week cumulative case rates, and found roughly the same pattern, albeit more scattered. This analysis, however, excludes counties that recorded fewer than eight cases over the previous two weeks. Most of those counties have small population sizes, and their vaccination rates range from the highest in the state to the lowest.

Vaccines being tested in Colorado for younger kids

Colorado’s most recent wave of coronavirus infections was driven by kids, including some too young to get vaccinated.

That wave is now subsiding, with case rates dropping among all age groups, Alden said. But Wednesday brought more good news for parents eager to get their younger kids immunized.

Children’s Hospital Colorado announced that it will be participating in clinical trials for the Pfizer coronavirus vaccine for kids ages 5 to 11. The study, which hopes to enroll 4,600 kids worldwide, will determine if the vaccine is safe and effective in kids that young. Currently, the Pfizer vaccine is approved for anyone ages 12 and older, and no coronavirus vaccine is approved for anyone younger than that.

Kids who will be part of the study have already been chosen in Colorado, using a lottery system.

“Children need a vaccine,” said Dr. Eric Simões, a doctor at Children’s who is leading the study there. “The vaccine trials get us one step closer to protecting our children and our communities from COVID-19.”