Omicron’s severity when compared to delta or other variants is still in doubt. What’s not in doubt is the rate of spread. So far in the United States, the cases of omicron are doubling every two days. That’s seven times faster than the spread of delta when that variant appeared. 

Omicron is rapidly displacing other variants in Europe and the United States, just as it did in South Africa. A portion of this is likely explained by a (not yet peer-reviewed) study from the University of Hong Kong which found that omicron replicates 70 times more quickly in the human respiratory system than did delta, or other variants. However, the level of infections in the lungs “is significantly lower than the original SARS-CoV-2, which may be an indicator of lower disease severity.” In fact, data suggests that omicron, while still present in the lungs, replicates 10 times more slowly there than does delta.

An extremely rapid replication in the bronchial system that has a decreased impact on the lungs helps to explain the spread of omicron in two ways. One, patients are breathing out a lot of viral particles because of the heavy concentration in airways. Two, patients are more likely to be highly contagious before symptoms appear due to the speed and location of the infection.

The data from Imperial College also suggests that omicron is largely evasive of past infection by delta or other variants. In fact, patients were found to be 5.4 times more likely to be reinfected by omicron, than they were to be reinfected by past variants. With just a 19% rate of protection from past infection, even patients recently recovered from delta are highly susceptible to becoming ill a second time with omicron. Whether past infection provides protection against severe illness is so far unclear.

As expected, omicron is also more evasive of vaccines. The results indicated that vaccine effectiveness in preventing symptomatic infection from omicron was between 0% and 20% for those more than two weeks after their second shot. Patients with a booster fared much better, with rates of protection between 55% and 80%. Those numbers cover the AstraZeneca vaccine as well as the Pfizer/BioNTech vaccine common in the United States.

Even if the South Africa data indication a two-thirds lower rate of death holds up, the extreme high rate of transmission by omicron, and its effectiveness in evading past infection and vaccination could mean that the United States is set to see new highs when it comes to rates of new cases over the following weeks. And while some are looking at the rapid rise and fall in South Africa as a sign that the omicron wave could be over as fast as it has set in, past experience with delta shows that minor differences in social distancing policy, vaccination rate, travel, and pure chance can greatly affect the scope and persistence of high infection peaks.

In any case, if the Imperial College data is right about rate of hospitalization from omicron, the incredible rate of spread suggests that hospitals in many parts of the U.S. are about to face another incredible round of stress. 

Beware of suggestions that omicron is “mild” or that social distancing rules can be relaxed over the holidays. Wear a mask. Avoid indoor spaces where large numbers of people are gathered. Order the take out. The rate of infectiousness suggested by the rapid spread of omicron is truly fierce—up there in the measles “I just walked through a room where someone infected walked through half an hour earlier, and I caught it” territory.

Do not make the assumption that omicron is a ticket to a mild case of COVID-19. This is a situation where the results of underestimating the threat can be much, much worse than overestimating.