New data is now available regarding the transmission of the SARS-CoV-2 Omicron variant. With regard to transmission, there are two major concerns. The first is how rapidly following infection people become infectious. The second is how long Omicron hosts remain infectious. Both of the concerns have been addressed by Hay et al., at least for healthy young men.

The study gathered PCR tests from basketball players in the National Basketball Association, aiming to elucidate the viral dynamics of Omicron, namely when viral load peaks and how long the general infection lasts. 

The NBA regularly tests players for Covid-19 to ensure the integrity of its 82-game season. Using the data collected from these samples, Hay et al. quantified the duration of viral proliferation, clearance rate, and peak viral concentration for individual infections. They additionally sequenced the samples to differentiate between Omicron, Delta, and other variants.

From their data set, Hay et al. noted 97 Omicron infections and 107 Delta infections. They found that 27 of the 97 Omicron cases tested positive one day or less after a previous negative. Of these, 52% were positive at day five, 25% at day six, and 13% at day seven. For the 70 of 97 testing positive two or more days after a previous negative, 39.1% were positive at day five, 33.3% at day six, and 22.2% at day seven.

FIGURE 1: PCR Ct value trajectories for each acute Omicron (red) and Delta (blue) infection. Solid colored lines show mean Ct value over time, counting negative tests as Ct=40. The horizontal dotted line shows the cutoff of PCR Ct=30 (legend from oriHAY ET AL.

This data illustrates that Omicron infected individuals become transmissible within the first few days of infection, with the viral load peaking rather early. The mean infection length for Omicron cases was 9.87 days. For comparison, Delta infections averaged over a day longer: 10.9 days. Peak viral load came slightly later in Delta cases as well, particularly in those that tested positive one or fewer days after testing negative, though there were only six such cases.

FIGURE 2: Mean posterior viral trajectories for Omicron (red) and Delta (blue) infections (legend from original study).HAY ET AL.

Hay et al. also found that the variant portrayed a lower peak viral load than the Delta variant. Omicron’s mean peak Ct value was 23.3 compared to Delta’s 20.5, emphasizing that the lower the Ct value, the higher the peak viral load. In other words, Delta infections elicit more virus particles in a host than Omicron infections, but only moderately. There is some speculation as to if the Ct value for Omicron should be higher than 30 based on its increased transmission advantage, but for the purposes of this study, the value for Omicron and Delta is set at 30.

FIGURE 3: (A) Peak viral RNA concentration, (B) proliferation time, (C) clearance time, and (D) acute infection duration for omicron (red) and delta (blue) infections (legend from original study).HAY ET AL.

Additionally, Hay et al. note that Omicron and Delta display similar rates of viral clearance, meaning how fast the virus is expelled from the host. As such, Omicron has a slightly shorter clearance phase, 5.35 days as compared to 6.23 days for Delta, as Omicron has a lower peak viral load.

These data points raise a sort of transmission paradox. If the virus is less concentrated in Omicron, infections are cleared faster and mean infection lengths are all lower than those of Delta, then why is Omicron so much more transmissible?

By some estimates, Omicron is 2.7 to 3.7 times more transmissible than the Delta variant. The first potential explanation is that protective convalescent antibodies from the previous infection display little to no protection against Omicron, whereas they provided some extended period of protection against Delta. While this may be the case, I believe it is only part of the story.

I believe Omicron is much more successful at transmission, in part, due to its immune-suppressive advantages. Once the virus enters the cell, Omicron may be more efficient at suppressing the innate immune response. The proteins which conduct interaction with innate immunity are those outside the Spike protein and mutations in these proteins may be the hidden key to Omicron’s proliferation.

We emphasize the importance of taking this strain seriously. The data indicate that peak viral loads and mean infection duration was greater for the Delta virus, but this is only half the picture. Omicron’s transmission advantage and potent immune suppression make it a real threat, especially as society continues to a pre-pandemic lifestyle, representing a breeding ground for the virus that will yield millions more infections and many deaths. Take caution, receive your vaccinations, and do not take Omicron for granted.