As delta variant surges, vaccines more important than ever

Dear Doctor: What’s the R-zero number that we’re hearing about with the delta variant, and why is it important? Also, I’m confused. If vaccinated people are getting COVID-19, why even bother to get vaccinated? Do the vaccines work or not? If they do, why would I still need a mask?

Dear Reader: You’ve asked some excellent questions, the complete answers to which could fill a few chapters in a medical text.

Let’s start with the R-zero, or R0 number. Also pronounced as “R naught,” it’s a mathematical term that shows how contagious a disease is. The “R” stands for reproduction and refers to the average number of people that someone with a transmissible disease will infect. An R0 of 1 means an infected person will transmit the disease to one other person, and that person will go on to infect another person. When an R0 is greater than 1, an outbreak will grow. When it’s less than 1, the number of infected people will decline. The flu has an R0 of between 1 and 2. That means someone with the flu will go on to infect one or two more people.

Based on the data available so far, the R0 of the original coronavirus that causes COVID-19 is between 1.5 and 4. For the highly more transmissible delta variant, the R0 number jumps to between 5 and 9. That puts it into the range of chicken pox, which has an R0 between 9 and 10. An R0 number is important because it lets us understand how much of a public health threat an infectious disease is and helps to guide the response.

Elizabeth Ko, MD and Eve Glazier, MD

The data that are being collected regarding the delta variant show that this version of the virus not only replicates more quickly than the original coronavirus, but it also produces far more virus particles than any of the previous variants. The significantly higher R0 number of the delta variant has affected the course of the pandemic.

It’s true that some vaccinated people may become infected with the delta variant, but the coronavirus vaccines protect them against serious, possibly life-threatening, illness. Unfortunately, the majority of the COVID-19 hospitalizations and deaths at this time are occurring in unvaccinated individuals.

Meanwhile, it has been shown that vaccinated individuals who become infected with the delta variant can also pass along the virus -- though they are infectious for a shorter time than those who are unvaccinated. This has led to a confusing and seemingly contradictory turning point in this pandemic. That is, you should absolutely get fully vaccinated as soon as possible, because the vaccines protect you against developing serious illness. And absolutely do continue to wear a mask when you’re spending time indoors in public areas. The delta variant’s R0 is so high, it shows us that this newer version of the coronavirus is far more contagious than the original. We recommend the use of the highly effective N-95 masks, which have once again become available to the general public. And please be patient. The virus continues to change, which means the guidance on staying safe will evolve as well.

(Send your questions to, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

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