Breakthrough Delta infections equally contagious in households.
New clinical data supports earlier findings linking viral loads and contagiousness.
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A new study in Lancet Infectious Diseases reports that breakthrough cases of the Delta variant of Covid-19 appear equally contagious as cases among the unvaccinated in household settings. Not great news. Not unexpected.
Last summer the Centers for Disease Control and Prevention warned that contagiousness of the Delta variant might be similar regardless of the vaccination status of a “source patient.” In other words, if someone was exposed to a person with an active Covid-19 illness, the risk of catching the virus did not hinge on whether the source was vaccinated. (The vaccines continued to decrease the odds that the exposed person would pick up an infection, and they still do, though Delta clearly lowered that level of protection). That information came in startling contrast to pre-Delta-era data which suggested that transmission from individuals with breakthrough infection was unusual.
In the new paper released Thursday, among household contacts living with a person with a known newly diagnosed symptomatic Covid-19 illness, 23-25% of these at-home potential targets eventually became infected. Non-household contacts who were followed had slightly lower (though “statistically similar”) rates of catching the virus. That “secondary attack rate” may sound low given how contagious Delta seems to be, but remember that the individuals who participated in this study were made aware that their household contact had an active case of Covid-19, took whatever precautions they could, and yet still became infected. If they had not known, it’s likely that the rates would have been far higher. That said, the household contacts were substantially less likely to become infected if they themselves had been fully vaccinated (most of the test subjects had been vaccinated with either the Pfizer-BioNTech or the AstraZeneca options, as the study took place in the United Kingdom last month), suggesting that the vaccines continued to provide some protection against infection, in addition to excellent protection against severe illness, hospitalization, and death.
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The new findings add much-needed if disappointing clinical data to earlier laboratory data that began to accumulate in July and August. Until now, the case for breakthrough contagion (or “forward transmission”) of the Delta variant being equal to that seen in unvaccinated cases has largely been circumstantial and indirect. The CDC’s initial concerns came from test results showing that unlike with previous variants, when nasal swabs taken from breakthrough and unvaccinated Delta patients were compared, the amount of viral genetic material was similar, implying that more contagious viral particles were present. Because the amount of virus a patient “sheds” from their mouths and noses is thought to correlate to levels of contagiousness—the higher the “viral load,” the more contagious the source patient is expected to be—these findings raised a red flag to public health officials, leading to changes in mask and other guidance to evolve with the changes Delta brought.
Based on this and other data, I began to believe that breakthrough cases with the Delta variant might be as contagious as unvaccinated cases at their peak, but perhaps for not as long. The new Lancet study did not address how long breakthrough cases were contagious, though they did track viral loads and found Delta viral loads increased more rapidly and declined more slowly than in infections with other variants. But in good news for the vaccines, viral loads dropped more quickly among breakthrough cases compared to unvaccinated ones, though the study did not attempt to measure whether these decreases in viral load correlated with fewer “secondary attacks” later in an illness. But based on these and previous data, I continue to believe that it’s likely that vaccination shortens the window of maximal contagiousness. Other studies are expected to assess that question.
If the correlation between lower viral loads and decreased contagiousness holds up, the fact that Delta viral loads dropped more quickly among the vaccinated may be another way of saying that the vaccines still continue to decrease spread, both by decreasing infections in the first place, and rendering those infections less contagious after a few days. Remember, the number of people a source patient infects depends on many factors—behavior being the main one (isolated people don’t spread respiratory viruses)—but if vaccinated people are maximally contagious for a few days less, the number of secondary attacks is likely to be lower.
Naturally, I wish the vaccines continued to stop most or all transmission from breakthrough infections. Nevertheless, these new findings should not dissuade anyone considering vaccination. In fact, any loss of confidence in the vaccines resulting from these findings reflects a self-inflicted wound by health officials and experts. Instead of focusing on the vaccines’ abilities to improve Covid-19 outcomes, keeping people alive and from preventing them from overwhelming hospitals, excessive attention has been paid to the vaccines’ temporary abilities to slow infection and spread. The fact that they do this at all temporarily is a tremendous gift, but was not a given. Experts like my friend and colleague Dr. Céline Gounder point out that potential oral and nasal vaccines are far more likely to provide a sustainable path towards decreasing infections down the road than the current ones. Other efforts like improving building air quality are also under-appreciated, despite 18 months of drumbeating by experts like Dr. Joseph Allen.
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In the spring, we were riding high with Covid-19 vaccines in the United States. Vaccination rates were on the rise while case counts, hospitalizations, and deaths were falling. All the data indicated that in addition to providing protection against severe illness and death, the vaccines also decreased the odds of infection, if only for a time. Protection against infection had never been guaranteed. In fact, the major clinical trials that established the efficacy of the Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines in preventing the worst outcomes from Covid-19 only followed infection rates as an afterthought.
We now know that early high levels of antibodies that result from vaccination naturally fall after a few months. (If they didn’t that could be a big problem). As a result, breakthrough infections are expected to be more likely, especially with variants like Delta which are slippery and evade our antibodies better than its ancestors. But because the vaccines train our immune system to respond to future SARS-CoV-2 infections, after a few days of mobilizing the troops, our bodies continue to mount an impressive fight. As we all know, a vast majority of the time, vaccinated individuals win those battles. So while clinical data now show the Delta variant spreading from individuals with breakthrough infection efficiently, we should continue to celebrate the fact that the implications of that spread are nothing like they were during the pre-vaccine era.
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