What you need to know about the Delta SARS-CoV-2 variant outbreak in Provincetown, Massachusetts

An outbreak of the Delta variant of SARS-CoV-2 in a widely vaccinated population in Provincetown, Massachusetts, fueled by public events and gatherings, was reported by the CDC on Friday.

What is concerning about this cluster is that a large part of the cases identified involved people who had been vaccinated with the Pfizer-BioNTech, Moderna or Johnson & Johnson vaccines.

The CDC report provides statistics on 469 cases that had been identified as of July 17. By July 20, according to NBC Boston, the total number of cases had risen to 882 cases.

There was significant transmission among fully vaccinated subjects

Of these 469 cases, 346 (74%) occurred in people who were fully vaccinated. These results show that transmission to vaccinated people can occur with sufficient regularity to cause an epidemic.

For comparison, the report notes that immunization coverage among eligible Massachusetts residents was 69% at the time of the outbreak. This should not be taken to mean that the vaccines were ineffective in preventing infection in this population. To estimate efficacy, we would need to know the fraction of the population actually exposed that was vaccinated. There are two reasons why an epidemic in the exposed population might not meet our expectations. First, there is geographic variation in immunization coverage within a state. Second, there will always be ‘breakthrough’ cases of infection, and as a population approaches full vaccination, an increasing fraction of cases will be among those vaccinated, giving a false impression of being vaccinated. efficiency. Third, there may be behavioral differences between vaccinated and unvaccinated people. In particular, if vaccinated people feel safer than unvaccinated people, they can be reasonably expected to engage in riskier behaviors, such as attending large gatherings.

Notably, all of the genetically identified cases belonged to the Delta variant. It is currently unclear whether the genetic makeup of the Delta variant was particularly important in triggering this epidemic.

Fully vaccinated people with symptomatic infections could transmit Delta just as much as unvaccinated people

Another key finding of the study is that there was no evidence of a difference in the “Ct value” for viral samples taken from vaccinated and unvaccinated people. “Ct” stands for “cycle threshold” and refers to the number of chemical reactions required to amplify the genetic material of the virus to make it detectable through a process called real-time reverse transcription PCR. This is important because the Ct value is generally correlated with viral load, which we know is an important contributor to transmissibility. At first glance, this suggests that vaccinated people, if infected and symptomatic, could transmit SARS-CoV-2 just as much as unvaccinated infected people. Importantly, since we do not know how many vaccinated people have been infected asymptomatically, we cannot draw firm conclusions about the effect of vaccination on transmission potential.

Does this mean that the vaccines are ineffective against the Delta variant?

In a nutshell – no. Vaccines are very effective in preventing death and serious illness. As of July 27, about 1% of Provincetown’s cases among those vaccinated had been hospitalized (just 4 of 374). This compares to a general hospitalization rate of around 10%. (I calculated this by looking at the total number of hospitalizations reported by the CDC on COVID-NET, multiplying by ten to reflect the total US population, and dividing by the current cumulative number of reported cases, which is just under 35 million.) Additionally, none of the 374 people have died, while the overall case fatality rate in the United States is currently around 1.7%.

In addition, there is no way to tell what the scale of the outbreak would have been if a smaller fraction of the population of Provincetown and Barnstable County had been vaccinated. An epidemic of several hundred people in a county of over 200,000 is relatively small, especially given the social conditions – “densely packed indoor and outdoor events in venues including bars, restaurants, guesthouses and rental homes ”- according to the CDC report.

Conclusion

The bottom line: There are some things we would really like to know, but just can’t learn from this data:

1. What is the effect of vaccination on the risk of becoming infected when exposed?
2. What is the effect of vaccination on the risk of transmission once infected?

Despite this, there are a number of key lessons we can learn.

1. Large gatherings remain important super-spreader events, even among the vaccinated.
2. Individual behavior is almost certainly linked to the risk of transmission. Dense gatherings of people should be avoided, even if they are fully immunized.
3. An epidemic can develop quickly. On July 3, the 14-day moving average number of cases in Barnstable County was zero. By the time the cases were over, it was too late to act. So the CDC’s new guideline that even vaccinated people should wear masks indoors in many places is wise advice.