Alpha, Delta and Now Omicron – 6 Critical Questions Answered As COVID-19 Cases

On Jan 23, 2022

The list of SARS-CoV-2 variants – each with its own unique qualities that give it an edge – just keeps growing.

“The Conversation” Editor’s note: The omicron variant of rapid surge in cases globally. We asked a team of virologists and immunologists from the University of Colorado Boulder to weigh in on some of the pressing questions that people are asking about the new variant.

How is omicron different from previous variants?

There are two key differences between omicron and previous variants of the SARS-CoV-2 virus that emerged in late 2019. Early data suggests that omicron cases are milder than infections caused by the delta variant. On the flip side, omicron is far more transmissible – meaning it spreads easier – than previous variants. It can be confusing to think about the overall effects of a milder virus that is also far more infectious.

When the delta variant became dominant and displaced alpha in the summer of 2021, it managed to do so because it was between 40% and 60% more transmissible. Now, the omicron variant is even more transmissible than delta.

It’s difficult to put numbers around how intrinsically more transmissible one variant is than another, because human behaviors and vaccination percentages are constantly in flux. Those factors, together with transmissibility, affect how a virus fares in a population.

In comparison with the original strain of SARS-CoV-2, omicron contains 72 mutations throughout its genome. Some of these mutations account for the complex new features that characterize this variant. Half of those changes are in the spike protein, the critical surface protein that enables the virus to latch on and infect cells. It is also the key virus feature that is recognized by the human immune system.

Why is omicron spreading so quickly?

Initial studies suggest that omicron is more effective at reproducing in the upper airways, including the nose, throat, and mouth, than earlier variants, making it more similar to a common cold virus. If data from these preliminary studies holds up, then it may help explain omicron’s high transmissibility: Viruses replicating in the upper airways may spread more easily, although the reasons for this are not completely understood.

In addition, omicron is often able to evade existing immunity long enough to start an infection, cause symptoms and transmit onward to the next person. This explains why reinfections and vaccine breakthrough infections seem to be more common with omicron.

Omicron contains a large number of mutations and is much more transmissible than earlier variants of the SARS-CoV-2 virus.

Those properties, and the timing of this variant emerging during the holiday season, resulted in the extraordinary surge in COVID-19 infections in the U.S. Add in wintertime – which brought people indoors – along with pandemic fatigue, and you have the perfect storm for rapid transmission.

The good news is that vaccination and vaccine boosters nevertheless provide good protection against severe disease and hospitalization. But given the current number of cases, that still means a lot of illnesses, hospitalizations and deaths in the weeks to come.

Could omicron move the population closer to herd immunity?

Herd immunity occurs when enough people have immunity to a virus that it no longer spreads well. It is only possible when two conditions are met. First, a large fraction of the population must be vaccinated or recovered from prior infection. Second, vaccination or prior infection must confer enough immunity to block or slow future infections. Will vaccination campaigns, combined with widespread omicron infection, be enough to bring herd immunity?

Three issues complicate the hope of achieving a long-term herd immunity after omicron. The first is that immunity naturally wanes over time, regardless of whether it comes from a vaccine or prior infection. It is not yet clear how long after infection or vaccination immunity to this virus lasts, since SARS-CoV-2 has been infecting humans for only two years. Eventually, controlled studies will be able to determine this.

Second, children younger than age 5 are not yet eligible for COVID-19 vaccines, and new susceptible children are born every day. So, until all age groups are eligible for vaccination, there will likely be ongoing transmission in kids.

And third, we…