英国研究：奥密克戎的再次感染风险是德尔塔的五倍
People walk across Westminster Bridge after new measures were announced due to the Omicron coronavirus variant, in London, Britain, November 28, 2021. /Reuters
12月16日，伦敦帝国理工学院发布了一份关于新冠病毒奥密克戎变异株的研究报告。
这项研究分析了11月29日至12月11日期间在英国英格兰经聚合酶链式反应（PCR）检测确诊为阳性的全部新冠病例。结果显示，奥密克戎的再感染风险比德尔塔毒株高5.41倍，且没有迹象表明这种毒株引发的症状比德尔塔更轻。
The risk of reinfection with the coronavirus variant Omicron is more than five times higher and it has shown no sign of being milder than Delta, a study showed, as cases soar across Europe and threaten year-end festivities.
The results of the study by Imperial College London (ICL) were based on UK Health Security Agency and National Health Service data on people who tested positive for COVID-19 in a PCR test in England between November 29 and December 11.
"We find no evidence (for both risk of hospitalization attendance and symptom status) of Omicron having different severity from Delta," the study said, although it added that data on hospitalizations remains very limited.
"Controlling for vaccine status, age, sex, ethnicity, asymptomatic status, region and specimen date, Omicron was associated with a 5.4-fold higher risk of reinfection compared with Delta," the study, which was dated December 16, added.
研究人员在一份声明中表示，曾感染新冠对预防奥密克戎毒株的有效性可能仅为19%，但同时也指出该研究尚未经过同行评审。
研究人员发现，与德尔塔毒株相比，那些在两周或两周以上接种过二剂疫苗和加强针的人，感染奥密克戎的风险显著增加。而即使如此，疫苗加强针依然会显著提高预防有效性，估计可预防55%至80%的有症状感染病例发生。
该研究的负责人尼尔·弗格森教授在声明中说：“这项研究提供了进一步的证据，表明在很大程度上奥密克戎可从之前由感染或疫苗产生的免疫中逃逸。这种程度的免疫逃逸能力意味着奥密克戎对公众健康构成了重大而紧迫的威胁。”
The protection afforded by past infection against reinfection with Omicron may be as low as 19 percent, ICL said in a statement, noting that the study had not yet been peer-reviewed.
The researchers found a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those who were two or more weeks past their second vaccine dose, and two or more weeks past their booster dose.
The study involved AstraZeneca and Pfizer vaccines.
Depending on the estimates used for vaccine effectiveness against symptomatic infection from the Delta variant, this translates into vaccine effectiveness of between zero and 20 percent after two doses, and between 55 percent and 80 percent after a booster dose.
"This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination," study lead Professor Neil Ferguson said in ICL's statement.
"This level of immune evasion means that Omicron poses a major, imminent threat to public health."
现在下结论还为时过早？
另一方面，英国疫苗工作组前主席克莱夫·迪克斯博士表示，重要的是不要过度解读这些数据。
“得出的结论是基于对奥密克戎的假设，而我们仍然没有足够的数据，”迪克斯博士说，“例如，我们没有细胞免疫反应方面的数据，而细胞免疫反应可以证明疫苗的有效性。这是模型中缺失的一个关键假设。”
他表示，一些结论与来自南非的数据不同——南非的疫苗目前在抵御重症和死亡方面表现良好。
But Dr Clive Dix, former Chair of the UK Vaccine Taskforce, said it was important not to overinterpret the data.
"The conclusions made are based on making assumptions about Omicron where we still don't have sufficient data," Dix said. "For example, we have no data on the cellular immune response which is now probably driving effectiveness of vaccines. This is a crucial missing assumption in the modeling."
Some of the conclusions are different from the data emerging from South Africa, where vaccines are holding up well against severe disease and death at present, he said.
"There is a huge amount of uncertainty in these modeled estimates and we can only be confident about the impact of boosters against Omicron when we have another month of real-world data on hospitalization ICU numbers and deaths," he said.
在奥密克戎出现之前，英国SIREN研究组对医务人员的再感染风险进行了研究，发现第一次冠状病毒感染在接下来的6个月里为第二次感染提供了85%的保护。
此次帝国理工学院分析的数据基于333000例病例，其中有德尔塔病例122062例和通过基因组测序确认为奥密克戎感染的病例1846例。
研究的共同领导者，帝国理工学院教授阿兹拉·加尼表示，这项研究“对于模拟奥密克戎未来可能的发展轨迹以及疫苗和其他公共卫生干预措施的潜在影响至关重要。”
这些新发现可能会促使一些欧洲国家实施更严格的限制，以阻止奥密克戎的传播。
An earlier study by Britain's SIREN looking at reinfection risk in health workers, which was carried out before Omicron emerged, found that a first coronavirus infection offered 85 percent protection from a second for the following six months.
The data analyzed by ICL was based on 333,000 cases, including 122,062 of Delta and 1,846 which were confirmed as the Omicron coronavirus variant through genome sequencing.
ICL's Professor Azra Ghani, who co-led the study, described it as "essential for modeling the likely future trajectory of the Omicron wave and the potential impact of vaccination and other public health interventions."
The new findings could accelerate the imposition of tighter restrictions across a number of European countries in a bid to stem the new variant's spread.